Quote:Gentle evolutionary change, sometimes even retrograde when desirable, not revolutionary.
As with most large and complex organisations over long periods of time, the development of the Roman army from the 2nd to the 5th centuries shows signs of both evolution and revolution, besides continuity.
The Simon James book I mentioned above does a good job of charting the evolutionary angle, as the Roman army transformed itself from a strategically and tactically aggressive force of conquest and expansion, into a microcosmic military society based on the frontiers, adapted for small-scale local defence and periodic large-scale punitive campaigns. Roman equipment and tactics evolved accordingly, to meet the requirements of different campaigns and different enemies.
James also talks of the 'Antonine Revolution' in military equipment (a phrase coined, I think, by Bishop and Coulston in 2006), when a changing political situation (the Marcomannic wars, mainly) prompted a series of local innovations and adaptations in kit and tactics. A monolithic frontier defence army had to develop an effective rapid-reaction field field, while maintaining its strength on the borders where possible. Most of these changes seem to have been 'bottom up', instituted by soldiers themselves or their immediate commanders, in response to immediate needs, rather than by some empire-wide decree, which makes it difficult for us to accurately trace them today.
The changes were further propelled by the Antonine Constitution of 212, which granted citizenship to all inhabitants of the empire, thereby erasing at a stroke the main distinction between legionaries and auxiliary soldiers. Unfortunately the paucity of sources for the next half century make it hard for us to judge the impact of this change - all we know if that army of the later 3rd and early 4th century appears in several critical ways to operate quite differently. Field armies are now composed of small legionary vexillations, commanded by tribunes or praepositi, often backed up by large numbers of 'barbarian' allies. Newer legions have appeared, often apparently quite a lot smaller than the older ones. The appearance of the Roman soldier on both imperial monuments and individual tombstones has changed radically.
The reign of Constantine (depending on your point of view) saw either another radical revolution in military affairs, or a official cementing of the changed situation already in place. The army was divided between frontier troops and field armies, which were duplicated between various different emperors and field commanders. Units were apparently even smaller, and there were a lot more of them. Many had new titles - a different form of auxilia, cavalry named 'equites' rather than alae, and a revised form of the old irregular 'numerus'.
Changed equipment suggests a change in battlefield tactics too - the late Roman 'fulcum', or shield-wall, may have developed from the old testudo, but appears to have been largely a defensive formation. Rather than the destructive pilum-volley followed by fast aggressive sword-charge that typifies (perhaps even defines) the military style of the principiate, we have a variety of considerably more static tactics, coupled with increased and sustained missile potential - a tactic of endurance and attrition, rather than the delivery of a sudden killer blow, perhaps reflecting both the increased sophistication of Rome's enemies and the increased frequency of civil wars.
At some point either at the end of Constantine's reign or during the reign of his sons, legion units were further split, or duplicated, by the division into seniores and iuniores (the earliest dated evidence is from the 350s).
But amid all this change, there was continuity, and perhaps more than appears immediately obvious. The frontier armies remained much the same, although their numbers dwindled. Equipment perhaps didn't suddenly alter - the pilum was still in use - in some places at least - into the late 3rd century and perhaps (under a different name?) later. The traditional rectangular shield seems to have survived in places into the 260s, and even the lorica segmentata may not have died out as rapidly as once thought - there are fragments from the later 3rd, perhaps even 4th centuries, so perhaps it was still in use then. In fact, a forthcoming popular title on the later Roman soldier might well make this latter suggestion more strongly...
So we know (I think) that change occured. The question of why it occured is harder to answer and leads us into speculation. The trouble is that most Roman commentators lacked the long-range historical view, and military focus, necessary to make sense of a series of changes probably even more baffling in detail than in general.
All armies are responses to the particular military needs of their era. The Roman army repeatedly proved itself able to adapt and to survive - in doing so, it demonstrated a flexible response to military and political necessity. Just as the Roman empire, and the world surrounding it, changed almost beyond recognition over the centuries, so did its army. A genuine insight into Roman military practice surely requires a balancing of the competing influences of evolution, revolution and continuity.