Military technology.  
The Roman military technology ranged from personal equipment and armament to deadly siege engines.  
Foot soldier.  
Weaponry.  
The pilum (javelin) was a weapon favored by legionaries and weighed approximately five pounds/2.25 kilograms. It was designed to be used only once and was destroyed upon initial use. This feature prevented the enemy from reusing spears. All soldiers carried two versions of this weapon: a primary spear and a backup. A solid block of wood in the middle of the weapon provided legionaries protection for their hands while carrying the device. According to Polybius, historians have records of "how the Romans threw their spears and then charged with swords". This tactic seemed to be common practice among Roman infantry.  
Armour.  
While heavy, intricate armour was not uncommon (cataphracts), the Romans perfected a relatively light, full torso armour made of segmented plates (lorica segmentata). This segmented armour provided good protection for vital areas but did not cover as much of the body as lorica hamata (chain mail). The lorica segmentata provided better protection, but the plate bands were expensive and difficult to produce and difficult to repair in the field. Generally, mail was cheaper, easier to produce, and simpler to maintain, was one-size-fits-all and was more comfortable to wear; thus, it remained the primary form of armour even when lorica segmentata was in use.  
Tactics.  
Testudo is a tactical military maneuver original to Rome. The tactic was implemented by having units raise their shields in order to protect themselves from enemy projectiles raining down on them. The strategy only worked if each member of the testudo protected his comrade. Commonly used during siege battles, the "sheer discipline and synchronization required to form a Testudo" was a testament to the abilities of legionnaires. Testudo, meaning tortoise in Latin, "was not the norm, but rather adopted in specific situations to deal with particular threats on the battlefield". The Greek phalanx and other Roman formations were a source of inspiration for this maneuver.  
Cavalry.  
The Roman cavalry saddle had four horns and is believed to have been copied from Celtic peoples.  
Siege warfare.  
Roman siege engines such as ballistas, scorpions and onagers were not unique, but the Romans were probably the first people to put ballistas on carts for better mobility on campaigns. On the battlefield, it is thought that they were used to pick off enemy leaders. There is one account of the use of artillery in battle from Tacitus, Histories III,23:
On engaging they drove back the enemy, only to be driven back themselves, for the Vitellians had concentrated their artillery on the raised road that they might have free and open ground from which to fire; their earlier shots had been scattered and had struck the trees without injuring the enemy. A ballista of enormous size belonging to the Fifteenth legion began to do great harm to the Flavians' line with the huge stones that it hurled; and it would have caused wide destruction if it had not been for the splendid bravery of two soldiers, who, taking some shields from the dead and so disguising themselves, cut the ropes and springs of the machine.  
In addition to innovations in land warfare, the Romans also developed the corvus (boarding device) a movable bridge that could attach itself to an enemy ship and allow the Romans to board the enemy vessel. Developed during the First Punic War it allowed them to apply their experience in land warfare on the seas.  
Ballistas and onagers.  
While core artillery inventions were notably founded by the Greeks, Rome saw opportunity in the ability to enhance this long-range artillery. Large artillery pieces such as carroballista and onagers bombarded enemy lines, before full ground assault by infantry. The manuballista would "often be described as the most advanced two-armed torsion engine used by the Roman Army". The weapon often looks like a mounted crossbow capable of shooting projectiles. Similarly, the onager, "named after the wild ass because of its 'kick'", was a larger weapon that was capable of hurling large projectiles at walls or forts. Both were very capable machines of war and were put to use by the Roman military.  
The Helepolis.  
The helepolis was a vehicle used to besiege cities. The vehicle had wooden walls to shield soldiers as they were transported toward the enemy's walls. Upon reaching the walls, the soldiers would disembark at the top of the 15 m tall structure and drop on to the enemy's ramparts. To be effective in combat, the helepolis was designed to be self-propelled. The self-propelled vehicles were operated using two types of motors: an internal motor powered by humans, or a counterweight motor powered by gravity. The human-powered motor used a system of ropes that connected the axles to a capstan. It has been calculated that at least 30 men would be required to turn the capstan in order to exceed the force required to move the vehicle. Two capstans may have been used instead of just the one, reducing the number of men needed per capstan to 16, for a total of 32 to power the helepolis. The gravity-powered counterweight motor used a system of ropes and pulleys to propel the vehicle. Ropes were wrapped around the axles, strung through a pulley system that connected them to a counterweight hanging at the top of the vehicle. The counterweights would have been made of lead or a bucket filled with water. The lead counterweight was encapsulated in a pipe filled with seeds to control its fall. The water bucket counterweight was emptied when it reached the bottom of the vehicle, raised back to the top, and filled with water using a reciprocating water pump, so that motion could again be achieved. It has been calculated that to move a helepolis with a mass of 40 metric tons, a counterweight with a mass of 1 metric ton was needed.
Greek fire.  
Originally an incendiary weapon adopted from the Greeks in 7th century AD, the Greek fire "is one of the very few contrivances whose gruesome effectiveness was noted by" many sources. Roman innovators made this already lethal weapon even more deadly. Its nature is often described as a "precursor to napalm". Military strategists often put the weapon to good use during naval battles, and the ingredients to its construction "remained a closely guarded military secret". Despite this, the devastation caused by Greek fire in combat is indisputable.  
Transportation.  
Pontoon bridge.  
Mobility, for a military force, was an essential key to success. Although this was not a Roman invention, as there were instances of "ancient Chinese and Persians making use of the floating mechanism", Roman generals used the innovation to great effect in campaigns. Furthermore, engineers perfected the speed at which these bridges were constructed. Leaders surprised enemy units to great effect by speedily crossing otherwise treacherous bodies of water. Lightweight crafts were "organized and tied together with the aid of planks, nails and cables". Rafts were more commonly used instead of building new makeshift bridges, enabling quick construction and deconstruction.  
Medical technology.   
Surgery.  
Although various levels of medicine were practised in the ancient world, the Romans created or pioneered many innovative surgeries and tools that are still in use today such as hemostatic tourniquets and arterial surgical clamps. Rome was also responsible for producing the first battlefield surgery unit, a move that paired with their contributions to medicine made the Roman army a force to be reckoned with. They also used a rudimentary version of antiseptic surgery years before its use became popular in the 19th century and had very capable doctors. 



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Transportation.  
Roads.  
The Romans primarily built roads for their military. Their economic importance was probably also significant, although wagon traffic was often banned from the roads to preserve their military value. In total, more than 400,000 kilometres (250,000 mi) of roads were constructed, 80,500 kilometres (50,000 mi) of which were stone-paved.  
Way stations providing refreshments were maintained by the government at regular intervals along the roads. A separate system of changing stations for official and private couriers was also maintained. This allowed a dispatch to travel a maximum of 800 kilometres (500 mi) in 24 hours by using a relay of horses.  
The roads were constructed by digging a pit along the length of the intended course, often to bedrock. The pit was first filled with rocks, gravel or sand and then a layer of concrete. Finally, they were paved with polygonal rock slabs. Roman roads are considered the most advanced roads built until the early 19th century. Bridges were constructed over waterways. The roads were resistant to floods and other environmental hazards. After the fall of the Roman Empire the roads were still usable and used for more than 1000 years.  
Most Roman cities were shaped like a square. There were 4 main roads leading to the center of the city, or forum. They formed a cross shape, and each point on the edge of the cross was a gateway into the city. Connecting to these main roads were smaller roads, the streets where people lived.  
Bridges.  
Roman bridges were built with stone and/or concrete and utilized the arch. Built in 142 BC, the Pons Aemilius, later named Ponte Rotto (broken bridge), is the oldest Roman stone bridge in Rome. The biggest Roman bridge was Trajan's Bridge over the lower Danube, constructed by Apollodorus of Damascus, which remained for over a millennium the longest bridge to have been built both in terms of overall and span length. They were most of the time at least 60 feet (18 m) above the body of water.  
Carts.  
Roman carts had many purposes and came in a variety of forms. Freight carts were used to transport goods. Barrel carts were used to transport liquids. The carts had large cylindrical barrels laid horizontally with their tops facing forward. For transporting building materials, such as sand or soil, the Romans used carts with high walls. Public transportation carts were also in use with some designed with sleeping accommodations for up to six people.  
The Romans developed a railed cargo system for transporting heavy loads. The rails consisted of grooves embedded into existing stone roadways. The carts used in such a system had large block axles and wooden wheels with metal casings.  
Carts also had brakes and elastic suspensions. The elastic suspension systems used leather belts attached to bronze supports to suspend the carriages above the axles. The system helped to create a smoother ride by reducing vibration. The Romans adopted bearings developed by the Celts. The bearings decreased rotational friction by using mud to lubricate stone rings.  
Industry.  
Mining.  
The Romans also made great use of aqueducts in their extensive mining operations across the empire, some sites such as Las Medulas in north-west Spain having at least 7 major channels entering the minehead. Other sites such as Dolaucothi in south Wales was fed by at least five leats, all leading to reservoirs and tanks or cisterns high above the present opencast. The water was used for hydraulic mining, where streams or waves of water are released onto the hillside, first to reveal any gold-bearing ore, and then to work the ore itself. Rock debris could be sluiced away by hushing, and the water also used to douse fires created to break down the hard rock and veins, a method known as fire-setting.  
Alluvial gold deposits could be worked and the gold extracted without needing to crush the ore. Washing tables were fitted below the tanks to collect the gold-dust and any nuggets present. Vein gold needed crushing, and they probably used crushing or stamp mills worked by waterwheels to comminute the hard ore before washing. Large quantities of water were also needed in deep mining to remove waste debris and power primitive machines, as well as for washing the crushed ore. Pliny the Elder provides a detailed description of gold mining in book XXXIII of his Naturalis Historia, most of which has been confirmed by archaeology. That they used water mills on a large scale elsewhere is attested by the flour mills at Barbegal in southern France, and on the Janiculum in Rome. 


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Waterworks.  
Sanitation.  
The Romans did not invent plumbing or toilets but instead borrowed their waste disposal system from their neighbors, particularly the Minoans. A waste disposal system was not a new invention, but rather had been around since 3100 BCE, when one was created in the Indus River Valley. The Roman public baths, or thermae served hygienic, social and cultural functions. The baths contained three main facilities for bathing. After undressing in the apodyterium or changing room, Romans would proceed to the tepidarium or warm room. In the moderate dry heat of the tepidarium, some performed warm-up exercises and stretched while others oiled themselves or had slaves oil them. The tepidarium's main purpose was to promote sweating to prepare for the next room, the caldarium or hot room. The caldarium, unlike the tepidarium, was extremely humid and hot. Temperatures in the caldarium could reach 40 degrees Celsius (104 degrees Fahrenheit). Many contained steam baths and a cold-water fountain known as the labrum. The last room was the frigidarium or cold room, which offered a cold bath for cooling off after the caldarium. The Romans also had flush toilets.  
Roman baths.  
The containment of heat in the rooms was important in the operation of the baths, as to avoid patrons from catching colds. To prevent doors from being left open, the door posts were installed at an inclined angle so that the doors would automatically swing shut. Another technique of heat efficiency was the use of wooden benches over stone, as wood conducts away less heat. 



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Engineering and construction.  
Wood.  
The Romans created fireproof wood by coating the wood with alum.  
Stone.  
It was ideal to mine stones from quarries that were situated as close to the site of construction as possible, to reduce the cost of transportation. Stone blocks were formed in quarries by punching holes in lines at the desired lengths and widths. Then, wooden wedges were hammered into the holes. The holes were then filled with water so that the wedges would swell with enough force to cut the stone block out of the Earth. Blocks with the dimensions of 69 by 14 by 15 ft (21.0 by 4.3 by 4.6 m) have been found, weighing about 1000 tons. There is evidence that saws were developed to cut stone in the Imperial age. Initially, Romans used saws powered by hand to cut stone but later went on to develop stone cutting saws powered by water.  
Cements.  
The mixture ratio of Roman lime mortars depended upon where the sand for the mixture was acquired. For sand gathered at a river or sea, the mixture ratio was two parts sand, one part lime, and one part powdered shells. For sand gathered further inland, the mixture was three parts sand and one part lime. The lime for mortars was prepared in limekilns, which were underground pits designed to block out the wind.  
Another type of Roman mortar is known as pozzolana mortar. Pozzolana is a volcanic clay substance located in and around Naples. The mixture ratio for the cement was two parts pozzolana and one part lime mortar. Due to its composition, pozzolana cement was able to form in water and has been found to be as hard as natural forming rock.  
Cranes.  
Cranes were used for construction work and possibly to load and unload ships at their ports, although for the latter use there is according to the "present state of knowledge" still no evidence. Most cranes were capable of lifting about 6–7 tons of cargo, and according to a relief shown on Trajan's Column were worked by treadwheel.  
Buildings.    
The Pantheon.    
The Romans designed the Pantheon thinking about the concepts of beauty, symmetry, and perfection. The Romans incorporated these mathematical concepts into their public works projects. For instance, the concept of perfect numbers was used in the design of the Pantheon by embedding 28 coffers into the dome. A perfect number is a number where its factors add up to itself. So, the number 28 is considered to be a perfect number, because its factors of 1, 2, 4, 7, and 14 add together to equal 28. Perfect numbers are extremely rare, with there being only one number for each quantity of digits (one for single digits, double digits, triple digits, quadruple digits, etc.). Embodying mathematical concepts of beauty, symmetry, and perfection, into the structure conveys the technical sophistication of Roman engineers.  
Roman concrete was essential to the design of the Pantheon. The mortar used in the construction of the dome is made up of a mixture of lime and the volcanic powder known as pozzolana. The concrete is suited for use in constructing thick walls as it does not require to be completely dry to cure.  
The construction of the Pantheon was a massive undertaking, requiring large quantities of resources and man-hours. Delaine estimates the amount of total manpower needed in the construction of the Pantheon to be about 400 000 man-days.    
Hagia Sophia.  
Although the Hagia Sophia was constructed after the fall of the Western empire, its construction incorporated the building materials and techniques of ancient Rome. The building was constructed using pozzolana mortar. Evidence for the use of the substance comes from the sagging of the structure's arches during construction, as a distinguishing feature of pozzolana mortar is the large amount of time it needs to cure. The engineers had to remove decorative walls to let the mortar cure.  
The pozzolana mortar used in the construction of the Hagia Sophia does not contain volcanic ash but instead crushed brick dust. The composition of the materials used in pozzolana mortar leads to increased tensile strength. A mortar composed of mostly lime has a tensile strength of roughly 200 kilopascals (30 psi) whereas pozzolana mortar using crushed brick dust has a tensile strength of 3,000 kilopascals (500 psi). The advantage of using pozzolana mortar in the construction of the Hagia Sophia is the increase in strength of the joints. The mortar joints used in the structure are wider than one would expect in a typical brick and mortar structure. The fact of the wide mortar joints suggests the designers of the Hagia Sophia knew about the high tensile strength of the mortar and incorporated it accordingly.  
Waterworks.  
Aqueducts.  
The Romans constructed numerous aqueducts to supply water. The city of Rome itself was supplied by eleven aqueducts made of limestone that provided the city with over one million cubic metres of water each day, sufficient for 3.5 million people even in modern times, and with a combined length of 350 kilometres (220 mi).  
Water inside the aqueducts depended entirely on gravity. The raised stone channels in which the water traveled were slightly slanted. The water was carried directly from mountain springs. After it had gone through the aqueduct, the water was collected in tanks and fed through pipes to fountains, toilets, etc.   
The main aqueducts in Ancient Rome were the Aqua Claudia and the Aqua Marcia. Most aqueducts were constructed below the surface with only small portions above ground supported by arches. The longest Roman aqueduct, 178 kilometres (111 mi) in length, was traditionally assumed to be that which supplied the city of Carthage. The complex system built to supply Constantinople had its most distant supply drawn from over 120 km away along a sinuous route of more than 336 km.  
Roman aqueducts were built to remarkably fine tolerances, and to a technological standard that was not to be equaled until modern times. Powered entirely by gravity, they transported very large amounts of water very efficiently. Sometimes, where depressions deeper than 50 metres had to be crossed, inverted siphons were used to force water uphill. An aqueduct also supplied water for the overshot wheels at Barbegal in Roman Gaul, a complex of water mills hailed as "the greatest known concentration of mechanical power in the ancient world".  
Roman aqueducts conjure images of water travelling long distances across arched bridges, however; only 5 percent of the water being transported along the aqueduct systems traveled by way of bridges. Roman engineers worked to make the routes of aqueducts as practical as possible. In practice, this meant designing aqueducts that flowed at ground level or below surface level, as these were more cost effective than building bridges, construction and maintenance for bridges was higher than that of surface and sub-surface elevations. Aqueduct bridges were often in need of repairs and spent years at a time in disuse. Water theft from the aqueducts was a frequent problem which led to difficulties in estimating the amount of water flowing through the channels. To prevent the channels of the aqueducts from eroding, a plaster known as opus signinum was used. The plaster incorporated crushed terracotta in the typical Roman mortar mixture of pozzolana rock and lime.  
Dams.  
The Romans built dams for water collection, such as the Subiaco Dams, two of which fed Anio Novus, one of the largest aqueducts of Rome. They built 72 dams in just one country, Spain and many more are known across the Empire, some of which are still in use. At one site, Montefurado in Galicia, they appear to have built a dam across the river Sil to expose alluvial gold deposits in the bed of the river. The site is near the spectacular Roman gold mine of Las Medulas. Several earthen dams are known from Britain, including a well-preserved example from Roman Lanchester, Longovicium, where it may have been used in industrial-scale smithing or smelting, judging by the piles of slag found at this site in northern England. Tanks for holding water are also common along aqueduct systems, and numerous examples are known from just one site, the gold mines at Dolaucothi in west Wales. Masonry dams were common in North Africa for providing a reliable water supply from the wadis behind many settlements.  
The Romans built dams to store water for irrigation. They understood that spillways were necessary to prevent the erosion of earth-packed banks. In Egypt, the Romans adopted the water technology known as wadi irrigation from the Nabataeans. Wadis were a technique developed to capture large amounts of water produced during the seasonal floods and store it for the growing season. The Romans successfully developed the technique further for a larger scale.  


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Ancient Roman technology.  
Ancient Roman technology is the collection of techniques, skills, methods, processes, and engineering practices which supported Roman civilization and made possible the expansion of the economy and military of ancient Rome (753 BC – 476 AD).  
The Roman Empire was one of the most technologically advanced civilizations of antiquity, with some of the more advanced concepts and inventions forgotten during the turbulent eras of Late Antiquity and the early Middle Ages. Gradually, some of the technological feats of the Romans were rediscovered and/or improved upon during the Middle Ages and the beginning of the Modern Era; with some in areas such as civil engineering, construction materials, transport technology, and certain inventions such as the mechanical reaper, not improved upon until the 19th century. The Romans achieved high levels of technology in large part because they borrowed technologies from the Greeks, Etruscans, Celts, and others.  
With limited sources of power, the Romans managed to build impressive structures, some of which survive to this day. The durability of Roman structures, such as roads, dams, and buildings, is accounted for in the building techniques and practices they utilized in their construction projects. Rome and its surrounding area contained various types of volcanic materials, which Romans experimented with in the creation of building materials, particularly cements and mortars. Along with concrete, the Romans used stone, wood, and marble as building materials. They used these materials to construct civil engineering projects for their cities and transportation devices for land and sea travel.  
Warfare was an essential aspect of Roman society and culture. The military was not only used for territorial acquisition and defense, but also as a tool for civilian administrators to use to help staff provincial governments and assist in construction projects. The Romans adopted, improved, and developed military technologies for foot soldiers, cavalry, and siege weapons for land and sea environments.  
In addition to military engineering, the Romans also made significant contributions to medical technologies.  
Types of power.  
Human power.  
The most readily available power sources to the ancients were human and animal. Mechanical devices were developed to assist in the manipulation of objects which exceeded human strength – one such device being the windlass, which used ropes and pulleys to manipulate objects. The device was powered by multiple people pushing or pulling on handspikes attached to a cylinder.  
Human power was also a factor in the movement of ships, particularly warships. Though wind-powered sails were the dominant form of power in water transportation, rowing was often used by military craft during battle engagements.  
Animal power.  
The primary usage of animal power was for transportation. Several species of animals were used for differing tasks. Being strong and cheap to maintain, oxen were used to farm and transport large masses of goods. If speed was desired, horses were utilized. The main environment which called for speed was the battlefield, with horses being used in the cavalry and scouting parties. For carriages carrying passengers or light materials donkeys or mules were generally used, as they were faster than oxen and cheaper on fodder than horses. Other than being used as a means of transportation, animals were also employed in the operation of rotary mills. Beyond the confines of the land, a schematic for a ship propelled by animals has been discovered. The work known as De rebus bellicis describes a ship powered by oxen. In this design oxen are attached to a rotary, moving in a circle on a deck floor, spinning two paddle wheels, one on either side of the ship. The likelihood that such a ship was ever built is low, due to the impracticality of controlling animals on a watercraft.  
Water power.   
Power from water was generated through the use of a water wheel. A water wheel had two general designs: the undershot and the overshot. The undershot water wheel generated power from the natural flow of a running water source pushing upon the wheel's submerged paddles. The overshot water wheel generated power by having water flow over its buckets from above. This was usually achieved by building an aqueduct above the wheel. Although it is possible to make the overshot water wheel 70 percent more efficient than the undershot, the undershot was generally the preferred water wheel. The reason being the economic cost of building an aqueduct was too high for the mild benefit of having the water wheel turn faster. The primary purpose of water wheels was to generate power for milling operations and to raise water above a system's natural height. Evidence also exists that water wheels were used to power saws, though only scant descriptions of such devices remain.  
Wind power.  
Wind power was used in the operation of watercraft, through the use of sails. Windmills do not appear to have been created in ancient times.  
Solar power.  
The Romans used the Sun as a passive solar heat source for buildings, such as bath houses. Thermae were built with large windows facing southwest, the location of the Sun at the hottest time of day.  
Theoretical types of power.  
Steam power.  
The generation of power through steam remained theoretical in the Roman world. Hero of Alexandria published schematics of a steam device that rotated a ball on a pivot. The device used heat from a cauldron to push steam through a system of tubes towards the ball. The device produced roughly 1500 rpm but would never be practical on an industrial scale as the labour requirements to operate, fuel and maintain the heat of the device would have come at too great a cost.  
Technology as a craft.  
Roman technology was largely based on a system of crafts. Technical skills and knowledge were contained within the particular trade, such as stonemasons. In this sense, knowledge was generally passed down from a tradesman master to a tradesman apprentice. Since there are only a few sources from which to draw upon for technical information, it is theorized that tradesmen kept their knowledge a secret. Vitruvius, Pliny the Elder and Frontinus are among the few writers who have published technical information about Roman technology. There was a corpus of manuals on basic mathematics and science such as the many books by Archimedes, Ctesibius, Heron (a.k.a. Hero of Alexandria), Euclid and so on. Not all of the manuals which were available to the Romans have survived, as lost works illustrate.  


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Collapse in the West and survival in the East (395–476 AD).  
The non-federated mobile field army, known as the comitatenses, was eventually split into a number of smaller field armies: a central field army under the emperor's direct control, known as the comitatensis palatina or praesentalis, and several regional field armies. Historians Santosuosso and Vogt agree that the latter gradually degraded into low-quality garrison units similar to the limitanei that they either supplemented or replaced. By the 5th century, a significant portion of Western Rome's main military strength lay in rented barbarian mercenaries known as foederati.  
As the 5th century progressed, many of the Empire's original borders had been either wholly or partially denuded of troops to support the central field army. In 395, the Western Roman Empire had several regional field armies in Italy, Illyricum, Gallia, Britannia and Africa, and about twelve border armies. By about 430, two more field armies were established in Hispania and Tingitania but the central government had lost control of Britannia as well as much of Gaul, Hispania, and Africa. In the same period, the Eastern Roman Empire had two palatine field armies (at Constantinople), three regional field armies (in the East, in Thrace, and in Illyricum) and fifteen frontier armies.  
As Roman troops were spread increasingly thin over its long border, the Empire's territory continued to dwindle in size as the population of the empire declined. Barbarian war bands increasingly began to penetrate the Empire's vulnerable borders, both as settlers and invaders. In 451, the Romans defeated Attila the Hun, but only with assistance from a confederation of foederatii troops, which included Visigoths, Franks and Alans. As barbarian incursions continued, some advancing as far as the heart of Italy, Rome's borders began to collapse, with frontier forces swiftly finding themselves cut off deep in the enemy's rear.  
Simultaneously, barbarian troops in Rome's pay came to be "in a condition of almost perpetual turbulence and revolt" from 409 onwards. In 476 these troops finally unseated the last emperor of the Western Roman Empire. The Eastern Roman forces continued to defend the Eastern Roman (Byzantine) Empire until its fall in 1453.  
The former Oxford University historian Adrian Goldsworthy has argued that the cause of the fall of the Roman Empire in the West should not be blamed on barbarization of the late Roman Army, but on its recurrent civil wars, which led to its inability to repel or defeat invasions from outside its frontiers. The East Roman or Byzantine empire on the other hand had fewer civil wars to contend with in the late fourth and early fifth centuries, or in the years from 383 to 432 A.D. 


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Comitatenses and limitanei (284–395 AD).  
A distinction between frontier guard troops and more mobile reserve forces had emerged with the use of certain troops to permanently man frontiers such as Hadrian's Wall in Britannia in the 2nd century AD. The competing demands of manned frontiers and strategic reserve forces had led to the division of the military into four types of troops by the early 4th century:
The limitanei or riparienses patrolled the border and defended the border fortifications. According to some older theories, the limitanei were "settled and hereditary" militia that were "tied to their posts." But according to most recent research, the limitanei were originally regular soldiers, including infantry, cavalry, and river flotillas, although they eventually became settled militia. According to Luttwak, by the time of Constantine I, the cunei of cavalry, and auxilia of infantry, both usually around 500 men strong, were local provincial units under sector commanders. According to Pat Southern and Karen Dixon, the legiones, auxilia, and cunei of the border armies were part of the limitanei, but higher-status than the older cohortes and alae which they had replaced.  
The comitatenses and the palatini were central field armies, usually stationed in the interior or rear areas of the empire as a strategic reserve. The permanent field armies of the palatini and comitatenses were expansions of the field escort of the emperors, which were larger than bodyguard units, becoming temporary field armies known as the sacer comitatus. The palatini were "praesental" armies, central field armies under the direct command of the emperors, while the comitatenses, were usually the regional field armies, although units could be moved between the two forces. The initial expansion of the emperor's escort units, although substantial, still did not form a large enough force to campaign independently until further expanded by Diocletian and Constantine I.  
The emperor Constantine I created the scholae to replace the old praetorian guard. The scholae were his personal guard and were mainly equipped as cavalry. Vogt suggests that the scholae formed two small central reserves (Latin: scholae) held to the strategic rear even of the comitatenses, one each in the presence of the emperors of West and East respectively. 
Of the four troop types, the limitanei (border guards) were once considered to have been of the lowest quality, consisting largely of peasant-soldiers that were both "grossly inferior" to the earlier legions and inferior also to their counterparts in the mobile field armies. However, more recent work establishes that the limitanei were regular soldiers.  
While the limitanei were supposed to deal with policing actions and low-intensity incursions, the duty of responding to more serious incidents fell upon the regional or provincial troops of the reduced field reserves of the comitatenses. The countering of the very largest scale incursions on a strategic scale was the task of the mobile field troops, the palatini and comitatenses diverted to strengthen the field armies and possibly accompanied by the emperor's scholae. Both border and field armies consisted of a mix of infantry and cavalry units although the weight of cavalry was, according to some authorities, greater in the mobile field armies. Overall, approximately one quarter of the army consisted of cavalry troops but their importance is uncertain. Older works such as the Eleventh Edition of the Encyclopædia Britannica (1911) state that the Roman military of the late Empire was "marked by that predominance of the horseman which characterised the earlier centuries of the Middle Ages," but many more recent authors believe that the infantry remained predominant.  
There is some dispute about whether this new military structure was put into place under the Emperor Diocletian or Constantine since both reorganised the Roman Army in the late 3rd and early 4th centuries to some degree. Both Diocletian and even his predecessor of thirty years Gallienus may already have controlled mobile strategic reserves to assist the empire's border forces; either Diocletian or Constantine expanded this nascent force into permanent field armies.  
Recruitment from amongst Roman citizens had become greatly curtailed as a consequence of a declining population, "cripplingly numerous" categories of those exempted from military service and the spread of Christianity with its pacifist message. Together, these factors culminated in "the withdrawal of the urban class from all forms of military activity." In their place, much of Rome's military were now recruited from non-Italian peoples living within the empire's borders. Many of these people were barbarians or semi-barbarians recently settled from lands beyond the empire, including several colonies of Carpi, Bastarnae and Sarmatians.  
Although units described as legiones existed as late as the 5th century in both the border and field armies, the legionary system was very different from that of the principate and early empire. Since the term legion continued to be used, it is unclear exactly when the structure and role of the legions changed. In the 3rd or 4th century, however, the legions' role as elite heavy infantry was substantially reduced and may have evaporated entirely. Instead, those "legions" that remained were no longer drawn exclusively (and perhaps hardly at all) from Roman citizens. Either Diocletian or Constantine reorganised the legions into smaller infantry units who, according to some sources, were more lightly armoured than their forebears. Their lighter armament may have been either because they "would not consent to wear the same weight of body armour as the legionaries of old" or, as in at least one documented instance, because they were prohibited from wearing heavy armour by their general in order to increase their mobility. 4th-century legions were at times only one sixth the size of early imperial legions, and they were armed with some combination of spears, bows, slings, darts and swords, reflecting a greater contemporary emphasis on ranged fighting. The auxilia and numeri had also largely disappeared. Constantine further increased the proportion of German troops in the regular army; their cultural impact was so great that even legionaries began wearing German dress. At the start of Diocletian's reign, the Roman army numbered about 390,000 men, but by the end of his reign he successfully increased the number to 581,000 men.  
Adoption of barbarian allies (358–395 AD).  
By the late 4th century, the Empire had become chronically deficient in raising sufficient troops from amongst its own population. As an alternative, taxation raised internally was increasingly used to subsidise growing numbers of barbarian recruits. The Romans had, for some time, recruited individual non-Roman soldiers into regular military units. In 358 AD, this practice was accelerated by the wholescale adoption of the entire Salian Franks people into the Empire, providing a ready pool of such recruits. In return for being allowed to settle as foederatii in northern Gallia on the near side of the Rhine, the Franks were expected to defend the Empire's borders in their territory and provide troops to serve in Roman units.  
In 376, a large band of Goths asked Emperor Valens for permission to settle on the southern bank of the Danube River on terms similar to the Franks. The Goths were also accepted into the empire as foederati; however, they rebelled later that year and defeated the Romans at the Battle of Adrianople. The heavy losses that the Roman military suffered during this defeat ironically forced the Roman Empire to rely still further on such foederati troops to supplement its forces. In 382, the practice was radically extended when federated troops were signed up en masse as allied contingents of laeti and foederatii troops separate from existing Roman units. Near-constant civil wars during the period 408 and 433 between various Roman usurpers, emperors and their supposed deputies such as Constantine III, Constantius III, Aetius and Bonifacius resulted in further losses, necessitating the handing over of more taxable land to foederati.  
The size and composition of these allied forces remain in dispute. Santosuosso argues that foederati regiments consisted mostly of cavalry that were raised both as a temporary levy for a specific campaign need and, in some cases, as a permanent addition to the army. Hugh Elton believes that the importance of foederati has been overstated in traditional accounts by historians such as A.H.M. Jones. Elton argues that the majority of soldiers were probably non-Italian Roman citizens, while Santosuosso believes that the majority of troops were almost certainly non-citizen barbari. 


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The army during the decline of the Empire.  
Barbarisation of the army (117–253 AD).  
By the time of the emperor Hadrian the proportion of Italians in the legions had fallen to just ten percent and provincial citizens now dominated. This low figure is probably a direct result of the changing needs of military staffing: a system of fixed border defences (Latin: limes) were established around the Empire's periphery under Hadrian, consolidating Trajan's territorial gains. These called for troops to be stationed permanently in the provinces, a prospect more attractive to locally raised rather than Italian troops. The higher prestige and pay to be found in the Italian dominated Praetorian Guard must also have played a role. The majority of the troops in the legions at the start of the 3rd century AD were from the more Romanised (though non-Italian) provinces, especially Illyria. As the century progressed, more and more barbarians (Latin: barbari) were permitted to settle inside of, and tasked with aiding in the defence of, Rome's borders. As a result, greater numbers of barbarous and semi-barbarous peoples were gradually admitted to the army.  
However, whether this regionalisation of the legions was partnered by a drop in the professionalism of the troops is contested. Antonio Santosuosso argues that the strict discipline and high motivation of the days of Marius had lapsed, but Andrew Alfoldi states that the Illyrian troops were both valiant and warlike, and Tacitus described German recruits as being natural mercenaries (Latin: vivi ad arma nati). It seems that discipline in the legions did slacken, with soldiers granted permission to live with wives outside of military lodgings and permitted to adopt a more lavish and comfortable lifestyle, in contrast to the strict military regimen of earlier years. However, it is by no means certain that this led to any reduction in the effectiveness of the legions, due to the greater ferocity and stature of the barbari recruits. The flavour of the Roman military, however, was now dictated by the increasing number of regional recruits, leading to a partial barbarisation of Rome's military forces beginning in this period. The barbarisation of the lower ranks was paralleled by a concurrent barbarisation of its command structure, with the Roman senators who had traditionally provided its commanders becoming entirely excluded from the army. By 235 AD the Emperor himself, the figurehead of the entire military, was a man born outside of Italy to non-Italian parents.  
The gradual inclusion of greater numbers of non-citizen troops into the military was taken a further step by the creation under Hadrian of a new type of force in addition to the legions and auxilia, known as numeri. Formed in bodies of around 300 irregular troops, the numeri were drawn from subjugate provinces and peoples of client-states or even from beyond the borders of the empire. They were both less regimented and less Romanised than auxiliary troops, with a "pronounced national character," including native dress and native war cries. The introduction of the numeri was a response to the need for cheap troops, who were nevertheless fierce and provided a force balance of light infantry and cavalry. They were therefore largely less well armed and less well trained than auxilia or legions, although more prestigious elite irregular native troops were also utilised. However, the legions still made up around one half of the Roman army at this point.  
Successive crises (238–359 AD).  
By the late Empire, enemy forces in both the East and West were "sufficiently mobile and sufficiently strong to pierce the Roman defensive perimeter on any selected axis of penetration"; from the 3rd century onwards, both Germanic tribes and Persian armies pierced the frontiers of the Roman Empire. In response, the Roman army underwent a series of changes, more organic and evolutionary than the deliberate military reforms of the Republic and early Empire. A stronger emphasis was placed upon ranged combat ability of all types, such as field artillery, hand-held ballistae, archery and darts. Roman forces also gradually became more mobile, with one cavalryman for every three infantrymen, compared to one in forty in the early Empire. Additionally, the Emperor Gallienus took the revolutionary step of forming an entirely cavalry field army, which was kept as a mobile reserve at the city of Milan in northern Italy. It is believed that Gallienus facilitated this concentration of cavalry by stripping the legions of their integral mounted element. A diverse range of cavalry regiments existed, including catafractarii or clibanarii, scutarii, and legionary cavalry known as promoti. Collectively, these regiments were known as equites. Around 275 AD, the proportion of catafractarii was also increased. There is some disagreement over exactly when the relative proportion of cavalry increased, whether Gallienus' reforms occurred contemporaneously with an increased reliance on cavalry, or whether these are two distinct events. Alfoldi appears to believe that Gallienus' reforms were contemporaneous with an increase in cavalry numbers. He argues that, by 258, Gallienus had made cavalry the predominant troop type in the Roman army in place of heavy infantry, which dominated earlier armies. According to Warren Treadgold, however, the proportion of cavalry did not change between the early 3rd and early 4th centuries.  
Larger groups of barbari began to settle in Rome's territories around this time, and the troops they were contracted to provide to the Roman army were no longer organised as numeri but rather were the forerunners of the later rented native armies known as federated troops (Latin: foederati). Though they served under Roman officers, the troops of these units were far more barbarised than the numeri, lacked Romanisation of either military structure or personal ideology, and were ineligible for Roman citizenship upon discharge. These native troops were not permitted to fight in native war bands under their own leaders, unlike the later foederati; instead, these troops were split into small groups attached to other Roman units. They existed therefore as a halfway house between numeri, who were encouraged to be Romanised, and the foederati, who raised officers from their own ranks and were almost entirely self-dependent. 


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The army at the height of the Empire.  
Imperial legions and reformation of the auxilia (27 BC – 117 AD).  
By the turn of the millennium, Emperor Augustus' primary military concern was to prevent Roman generals from further usurping the imperial throne. The experience of Caesar and, earlier, Marius and Sulla, had demonstrated the willingness of "emergency" (re-activated previously decommissioned) legions containing troops keen for plunder to follow their generals against the state. Augustus therefore removed the need for such emergency armies by increasing the size of the standing armies to a size sufficient to provide territorial defence on their own. Perhaps due to similar concerns, the legions and auxiliaries of the army were supplemented under the Emperor Augustus by an elite formation of guards dedicated to the protection of the Emperor. The first such unit was based in Rome and were known as the Praetorian Guard, and a second similar formation were known as the Cohortes urbanae.  
The legions, which had been a mix of life professionals and civilian campaigners, was altered into a standing army of professionals only. The actual structure of the cohort army remained much the same as in the late Republic, although around the 1st century AD the first cohort of each legion was increased in size to a total of 800 soldiers. However, while the structure of the legions remained much the same, their make-up gradually changed. Whereas early Republican legions had been raised by a draft from eligible Roman citizens, imperial legions were recruited solely on a voluntary basis and from a much wider base of manpower. Likewise, whereas Republican legions had been recruited almost exclusively in Italy, early Imperial legions drew most of their recruits from Roman colonies in the provinces from 68 AD onwards. One estimate place the proportion of Italian troops at 65% under Augustus in c. 1 AD, falling to around 49% by the end of Nero's reign.  
Since the legions were officially open only to Roman citizens, Max Cary and Howard Hayes Scullard argue that at least in some provinces at this time "many provincials must have been recruited who lacked any genuine claim to Roman citizenship but received it unofficially on enlistment," a practice that was to increase in the 2nd century. This is most likely in those provinces where the pool of Roman citizens was not large enough to fulfil the provincial army's recruitment needs. One possible example is Britain, where one estimate puts the citizen pool in the 1st century at only 50,000 out of a total provincial population of around two million.  
At the same time as the legions underwent these transformations, the auxilia were reorganized and a number of allied troops were formalised into standing units similar to legions. Rather than being raised re-actively when required, the process of raising auxiliary troops was carried out in advance of conflicts according to annual targets. Whereas the internal organisation of the auxilia had previously been left up to their commanders, in the early empire they were organised into standardised units known as turmae (for cavalry alae) and centuriae (for infantry cohortes). Although never becoming as standardised in their equipment as the legions, and often retaining some national flavour, the size of the units at least was standardised to some degree. Cavalry were formed into either an ala quingenaria of 512 horsemen, or an ala millaria of 1,000 horsemen. Likewise, infantry auxilia could be formed into a cohors quingenaria of 500 men or a cohors millaria of 1,000 men. Mixed cavalry/infantry auxiliaries were typically formed with a larger proportion of foot than horse troops: the cohors equitata quingenaria consisted of 380 foot and 120 horsemen, and the cohors equitata millaria consisted of 760 foot and 240 horsemen.   
The vitality of the empire at this point was such that the use of native auxilia in the Roman army did not apparently barbarise the military as some scholars claim was to happen in the late empire. On the contrary, those serving in the auxilia during this period frequently strove to Romanise themselves. They were granted Roman citizenship on retirement, granting them several social advantages, and their sons became eligible for service in the legions.  
As with the army, many non-Italians were recruited into the Roman Navy, partly because the Romans had never readily taken to the sea. It appears that the navy was considered to be slightly less prestigious than the auxilia but, like the auxilia, troops could gain citizenship on discharge upon retirement. In terms of structure, each ship was staffed by a group of men approximately equivalent to a century, with ten ships forming a naval squadron.  
Introduction of vexillationes (76–117 AD).  
Through the final years of the 1st century AD, the legions remained the backbone of the Roman army, although the auxilia in fact outnumbered them by up to half as much again. Within the legions, the proportion of troops recruited from within Italy fell gradually after 70 AD. By the close of the 1st century, this proportion had fallen to as low as 22 percent, with the remainder drawn from conquered provinces. Since technically only citizens were allowed to enlist in the legions, where recruits did not possess citizenship then, at least in some instances, citizenship "was simply given [to] them on enlistment". During this time, the borders of the Empire had remained relatively fixed to the extent originally reached under the Emperor Trajan. Because of this, the army was increasingly responsible for protecting existing frontiers rather than expanding into foreign territory, the latter of which had characterised the army's earlier existence. As a result, legions became stationed in largely fixed locations. Although entire legions were occasionally transferred into theatres of war, they remained largely rooted in one or more legionary bases in a province, detaching into smaller bodies of troops (Latin: vexillationes) on demand. This policy eventually led to a split of the military's land-based forces into mobile and fixed troops in the later Empire. In general, the best troops were dispatched as vexillationes, and the remainder left to guard border defenses were of lower quality, perhaps those with injuries or near retirement. 


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Marian legion (107–27 BC).  
Modern historiography has regularly cast Marius as abolishing the propertied militia and replacing it with landless soldiers motivated largely by pay. This belief emerges from the ancient literary sources, but rests on a relatively weak basis. 
Despite enrolling some three to five thousand volunteers during the Jugurthine War, Gaius Marius assumed command of consular legions recruited via hitherto normal procedure in the following Cimbric War. Conscription continued after Marius's time, especially during the Social War, and the wealth and social background of the men who joined before and after the opening of recruitment changed little. There is little evidence that later Roman armies during the 1st century BC were made up of volunteers; almost all ancient references to army recruitment, outside private armies, involve conscription.  
For much of the 20th century, historians held that the property qualification separating the five classes and the capite censi was reduced over the course of the second century to a nugatory level due to a shortage of manpower. The basis for that belief, however, was merely three undated Roman figures for the amount of property required to serve which would serve as evidence for reductions only if forced into a descending order. Many scholars have also now abandoned the notion that Italy suffered in the second century BC any deficit of manpower which would have driven such putative reductions.  
Modern historians have also sometimes credited to Marius the abolition of Roman cavalry and light infantry and their replacement with auxilia. There is no direct evidence for this contention, which is driven largely by literary sources' silence on those branches after the 2nd century; continued inscriptional evidence attests both citizen cavalry and light infantry into the end of the republic. The decline of Roman light infantry has been connected not to reform but cost. Because the logistical cost of supporting light infantry and heavy infantry was relatively similar, the Romans chose to deploy heavy infantry in extended and distant campaigns due to their greater combat effectiveness, especially when local levies could substitute for light infantry brought from Rome and Italy.  
The changes to the Roman army during the 1st century BC are now more attributed to the Social War and the civil wars from 49 to 31 BC. The large-scale downsizing of Roman cavalry detachments likely emerged from the extension of citizenship to all of Italy. Because Italy's enfranchisement meant that Rome was now directly liable for the cavalry's upkeep rather than their local communities, Rome instead levied auxilia from allies who, by treaty, were responsible for their contingents' upkeep.  
Pay remained extremely low – only five asses per day – and irregular. Moreover, although the surviving sources frequently characterise soldiers as "poor", these sources largely reflect the perspectives of the elite, by whom the vast majority of the population were considered "poor" and for whom poverty needed not entail actual landlessness. Many of the soldiers of the 1st century BC possessed modest lands. Nor did the legions meaningfully professionalise: as, in general, both soldiers and commanders served only for short periods intending, respectively, to secure plunder or political advancement from military victory.  
After the Social War, the state also started to keep men under arms for longer periods to maintain available experienced manpower, and coupled this with longer terms for commanders, particularly Caesar and Pompey. Client armies emerged but not in the 100s BC but rather in the decades before Caesar's civil war, which broke out in 49 BC.  
The legions of the late Republic were, structurally, almost entirely heavy infantry. The legion's main sub-unit was called a cohort and consisted of approximately 480 infantrymen. The cohort was therefore a much larger unit than the earlier maniple sub-unit and was divided into six centuriae of 80 men each. Each centuria was separated further into 10 "tent groups" (Latin: contubernia) of 8 men each. Legions additionally consisted of a small body, typically 120 men, of Roman legionary cavalry (Latin: equites legionis). The equites were used as scouts and dispatch riders rather than battlefield cavalry. Legions also contained a dedicated artillery crew of perhaps 60 men, who would operate devices such as ballistae.  
Each legion was normally partnered with an approximately equal number of allied (non-Roman) auxiliae troops. The addition of allied troops to the Roman army was a formalisation of the earlier arrangement of using light troops from the Socii and Latini, who had received Roman citizenship after the Social War. Auxiliary troops could be formed from either auxiliary light cavalry known as alae, auxiliary light infantry known as cohors auxiliae, or a flexible mixture of the two known as cohors equitata. Cavalry types included mounted archers (Latin: sagittarii) and heavy shock cavalry (Latin: cataphracti or clibanarii). Infantry could be armed with bows, slings, throwing spears, long swords, or thrusting spears. Auxiliary units were originally led by their own chiefs, and, in this period, their internal organisation was left to their commanders.  
However, "the most obvious deficiency" of the Roman army remained its shortage of cavalry, especially heavy cavalry; even auxiliary troops were predominantly infantry. Luttwak argues that auxiliary forces largely consisted of Cretan archers, Balearic slingers and Numidian infantry, all of whom fought on foot. As Rome's borders expanded and its adversaries changed from largely infantry-based to largely cavalry-based troops, the infantry-based Roman army began to find itself at a tactical disadvantage, particularly in the East.  
After having declined in size following the subjugation of the Mediterranean, the Roman navy underwent short-term upgrading and revitalisation in the late Republic to meet several new demands. Under Caesar, an invasion fleet was assembled in the English Channel to allow the invasion of Britain; under Pompey, a large fleet was raised in the Mediterranean Sea to clear the sea of Cilician pirates. During the civil war that followed, as many as a thousand ships were either constructed or pressed into service from Greek cities.  
Non-citizen recruitment (49–27 BC).  
By the time of Julius Caesar in 54 BC, regular legionary units were supplemented by exploratores, a body of scouts, and speculatores, spies who infiltrated enemy camps. Due to the demands of the civil war, the extraordinary measure of recruiting legions from non-citizens was taken by Caesar in Transalpine Gaul (Latin: Gallia Transalpina), by Brutus in Macedonia, and by Pompey in Pharsalus. This irregular and extraordinary recruitment was not, however, typical of recruitment during this period, and Roman law still officially required that legions were recruited from Roman citizens only.  


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