Roman Legion in late Antiquity

[Nathan Ross]

I believe that the change in tactics and strategy from offensive to defensive warfare played a major part in the reforms of the fourth century, based both on the change to defence and the focus changing from attacking Rome's enemies to defending against internal rebellion and not incurring large numbers of casualties on the army.

I just want to pick up on this offensive/defensive point again, as it's interesting, I think.

Leaving aside tactics (it does seem that the battlefield tactics of the later army stressed defensive formations, shield-walls etc, rather than the offensive sword-charges of earlier periods), the Roman army was essentially defensive from the principiate onwards. After Trajan there were few wars of conquest beyond the borders, and Hadrian's policies seemed to echo those of Augustus - a strong frontier manned by legions in permanent bases, rather than aggressive mobility and expansion.

This policy paid off until the series of crises in the late Antonine period, when the large scale movement of legions from the northern frontiers to the east for Verus's Parthian war, coupled with what appears to be increasing cooperation among the northern barbarian peoples (part of a process that led to the great 'confederations' of Franks, Alamanni and Goths) led to devastating invasions and a defensive war that lasted for over a decade.

The use of mobile detachments, which really seems to have taken off during the Marcomannic war, was therefore a way to put together strong 'offensive' field armies without depleting the 'defensive' strength of the legions on the borders, thereby maintaining security over a wide stretch of frontier while allowing for rapid reaction against incursions and reprisal strikes into barbarian territory.

Whether it was Aurelian (with his 'select army' in the east) or Diocletian who brought this system to perfection we can't tell, but in it, I think, lies the seed of all the future developments in army structure into the 4th century, and the eventual institution of the two-tier army of comitatensis and limitanei legions and auxilia, perhaps each maintaining a different internal organisation to reflect their different purpose. This new army, in turn, allowed Constantine and his successors to both maintain the security of the frontiers and mount concerted offensive actions into enemy territory - Constantine's partial reconquest of Dacia in the 330s, for example, was the first dedicated 'imperial expansion' for over a century.