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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