Page "Roman army" Paragraph 44

By the end of Augustus ' reign, the imperial army numbered some 250, 000 men, equally split between legionaries and auxiliaries 25 legions and c. 250 auxiliary regiments ).

The numbers grew to a peak of about 450, 000 by 211 ( 33 legions and c. 400 auxiliary regiments ).

By then, auxiliaries outnumbered legionaries substantially.

From the 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 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 – 38 ).