Early Roman army.  
Tribal forces (c. 752 BC – c. 578 BC).  
According to the historians Livy and Dionysius of Halicarnassus, writing at a far later date, the earliest Roman army existed in the 8th century BC. During this period Rome itself was probably little more than a fortified hilltop settlement and its army a relatively small force, whose activities were limited "mainly [to] raiding and cattle rustling with the occasional skirmish-like battle". Historian Theodor Mommsen referred to it as Rome's curiate army, named for its presumed subdivision along the boundaries of Rome's three founding tribes (Latin: curiae)), the Ramnians, Tities and Luceres. This army's exact structure is not known, but it is probable that it loosely resembled a warrior band or group of bodyguards led by a chieftain or king.  Mommsen believes that Roman military organization of this period was regimented by the "Laws of the apocryphal] King Italus" but these laws, though referred to by Aristotle, have been lost.  
The army (Latin: legio) consisted, according to Livy, of exactly 3,000 infantry and 300 horsemen, one third from each of Rome's three founding tribes. Warriors served under six "leaders of division" (Latin: tribuni) who in turn served under a general, usually in the person of the reigning King. Mommsen uses philological arguments and references from Livy and others to suggest that the greater mass of foot-soldiers probably consisted of pilumni (javelin-throwers), with a smaller number possibly serving as arquites (archers). The cavalry was far smaller in number and probably consisted solely of the town's richest citizens. The army may also have contained the earliest form of chariots, hinted at by references to the flexuntes ("the wheelers").  
By the beginning of the 7th century BC, the Iron-Age Etruscan civilization (Latin: Etrusci) was dominant in the region. Like most of the other peoples in the region, the Romans warred against the Etruscans. By the close of the century, the Romans had lost their struggle for independence, and the Etruscans had conquered Rome, establishing a military dictatorship, or kingdom, in the city.  
Etruscan-model hoplites (578 BC – c. 315 BC).  
Although several Roman sources including Livy and Polybius talk extensively about the Roman army of the Roman Kingdom period that followed the Etruscan capture of the city, no contemporary accounts survive. Polybius, for example, wrote some 300 years after the events in question, and Livy some 500 years later. Additionally, what records were kept by the Romans at this time were later destroyed when the city was sacked. The sources for this period cannot therefore be seen as reliable, as they can be for later military history, e.g. from the First Punic War onwards.  
According to our surviving narratives, the three kings of Rome during the Etruscan occupation were Tarquinius Priscus, Servius Tullius, and Tarquinius Superbus. During this period the army underwent a reformation into a centurial army based on socio-economic class. This reformation is traditionally attributed to Servius Tullius, the second of the Etruscan kings. Tullius had earlier carried out the first Roman census of all citizens. Livy tells us that Tullius reformed the army by transplanting onto it the structure derived originally for civil life as a result of this census. At all levels, military service was, at this time, considered to be a civic responsibility and a way of advancing one's status within society.
However, Rome's social classes were qualified rather than created by the census. It is perhaps more accurate to say therefore that the army's structure was slightly refined during this period rather than radically reformed. Prior to these reforms, the infantry was divided into the classis of rich citizens and the infra classem of poorer citizens. The latter were excluded from the regular line of battle on the basis that their equipment was of poor quality. During the reforms, this crude division of poorer and richer citizens was further stratified. The army thereafter consisted of a number of troop types based upon the social class of propertied citizens, collectively known as adsidui. From the poorest in the "fifth class" to the richest in the "first class" and the equestrians above them, military service was compulsory for all. However, Roman citizens at this time generally viewed military service as a proper undertaking of duty to the state, in contrast to later views of military service as an unwelcome and unpleasant burden.  Whereas there are accounts of Romans in the late empire mutilating their own bodies in order to exempt themselves from military service, there seems to have been no such reluctance to serve in the military of early Rome. This may in part be due to the generally lower intensity of conflict in this era; to the fact that men were fighting close to and often in protection of their own homes, or due to—as posited by later Roman writers—a greater martial spirit in antiquity.  
The equestrians, the highest social class of all, served in mounted units known as equites. The first class of the richest citizens served as heavy infantry with swords and long spears (resembling hoplites) and provided the first line of the battle formation. The second class were armed similarly to the first class, but without a breastplate for protection, and with an oblong rather than a round shield. The second class stood immediately behind the first class when the army was drawn up in battle formation. The third and fourth classes were more lightly armed and carried a thrusting-spear and javelins. The third class stood behind the second class in battle formation, normally providing javelin support. The poorest of the propertied men of the city comprised the fifth class. They were generally too poor to afford much equipment at all and were armed as skirmishers with slings and stones. They were deployed in a screen in front of the main army, covering its approach and masking its manoeuvres.  
Men without property, who were thereby excluded from the qualifying social classes of the adsidui, were exempted from military service on the grounds that they were too poor to provide themselves with any arms whatsoever. However, in the most pressing circumstances, even these proletarii were pressed into service, though their military worth was probably questionable. Troops in all of these classes would fight together on the battlefield, with the exception of the most senior troops, who were expected to guard the city.  
The army is said to have increased from 3,000 to 4,000 men in the 5th century BC, and then again from 4,000 to 6,000 men sometime before 400 BC. This later army of 6,000 men were then divided into 60 centuries of 100 men each.  


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