Roman Imperial army

Imperial Roman Army
Roman infantry helmet (Carnuntum Auxiliary B). Late 1st century
Active	30 BC–284 AD
Country	Roman Empire
Branch	Army
Size	450,000 at peak in 211 AD
Unit types	Roman legions, Auxilia, Praetorian Guard
Disbanded	Became the late Roman army

The Imperial Roman army is a term that may be applied to the terrestrial armed forces deployed by the Roman Empire from about 30 BC to 476 AD. This period is sometimes spilt into the Principate (30 BC - 284 AD) and Dominate (285-476) periods.

Under Augustus (ruled 30 BC – 14 AD), the army consisted of legions, eventually auxilia and also numeri.

As all-citizen formations, and symbolic protectors of the dominance of the Italian "master-nation", legions enjoyed greater social prestige than the auxilia for much of the Principate. This was reflected in better pay and benefits. In addition, legionaries were equipped with more expensive and protective armour than auxiliaries, notably the lorica segmentata, or laminated-strip armour. However, in 212, the Emperor Caracalla granted Roman citizenship to nearly all the Empire's freeborn inhabitants. At this point, the distinction between legions and auxilia became moot, the latter becoming all-citizen units also. The change was reflected in the disappearance, during the 3rd century, of legionaries' special equipment, and the progressive break-up of legions into cohort-sized units like the auxilia.

By the end of Augustus' reign, the imperial army numbered some 250,000 men, equally split between 25 legions and 250 units of auxiliaries. The numbers grew to a peak of about 450,000 by 211, in 33 legions and about 400 auxiliary units. By then, auxiliaries outnumbered legionaries substantially. From this peak, numbers probably underwent a steep decline by 270 due to plague and losses during multiple major barbarian invasions. Numbers were restored to their early 2nd-century level of c. 400,000 (but probably not to their 211 peak) under Diocletian (r. 284-305). After the Empire's borders became settled (on the Rhine-Danube line in Europe) by AD 68, virtually all military units (except the Praetorian Guard) were stationed on or near the borders, in roughly 17 of the 42 provinces of the empire in the reign of Hadrian (r. 117–138).