List of Roman legions

The remaining 28 legions became the core of the early Imperial army of the Principate (27 BC – AD 284), most lasting over three centuries.

During the Dominate period (near the end of the Empire, 284–476), legions were also professional, but are little understood due to scarcity of evidence compared to the Principate.

What is clear is that late legions were radically different in size, structure, and tactical role from their predecessors, despite several retaining early period names.

Republican legions were formed by compulsory levy of Roman citizens (who met a minimum property qualification) and raised whenever it was necessary.

Throughout the late Republic, soldiers and commanders served only for short periods intending, respectively, to secure plunder or political advancement from military victory.

[2] There is little evidence of changes in the wealth and social backgrounds of the men who joined before and after the opening of recruitment;[3] the pay remained pitiful.

[5] The evidence for the abolition of property requirements is very weak,[6][7] and there were no significant changes in population size or demographics that would drive such a reduction.

Their command of standing legions in distant and arduous military campaigns resulted in the allegiance of those units transferring from the Roman state to themselves.

These imperatores (lit: victorious generals, from the title imperator they were hailed with by their troops) frequently fell out with each other and started civil wars to seize control of the state, such as Sulla, Caesar, Pompey, Crassus, Mark Antony and Octavian (later Augustus, the first Emperor himself).

Province names and borders are assumed throughout the Principate period as at 107, during the rule of Trajan, and after the annexation of Dacia and Arabia Petraea.

In reality provincial borders were modified several times between 30 BC and 284: this explains any discrepancy with other sources, as to a legion's location at a particular date.

Diocletian reorganized the Roman army, in order to better counter the threat of the Germanic peoples of northern Europe as well as that of the Persians from the East.