The Roman Army AD 250-378

Between the reigns of Augustus and Trajan the Roman Army perhaps reached its pinnacle. It is the army of this time which is generally understood as the 'classical' Roman army. However, contrary to popular belief, this was not the army which was eventually defeated by the northern barbarians.
The Roman army evolved, changing in time, adapting to new challenges. For a long time it didn't need to change much as it held supremacy on the battlefield. And so until AD 250 it was still the heavy armed infantry which dominated the Roman army.
But the day of gladius and the pilum were eventually to become a thing of the past.
The main reason for such changes to come about were the demands border warfare was placing on the army.
From the time of Hadrian onwards defensive systems along the Rhine Danube and Euphrates held off the opponents with large permanent camps placed along these boundaries. Any barbarians who crossed the border would need to make his way across the defences and locally stationed auxiliary forces only to eventually face the nearest legion which would march up from its camp and cut off their retreat. For a long time this system worked well enough.
But in the third century it could no longer cope. The old legions became gradually more disorganized, having cohorts detached and sent to various places to fill breeches in the defences.
A whole host of new cavalry and infantry units had been created in desperate times of civil war and barbarian invasions.
One of the most significant differences between the old army system was that Caracalla in AD 212 had bestowed Roman citizenship on all the provinces. With this the ancient distinction between the legionaries and the auxiliary forces had been swept aside, each now being equal in their status.
So provincial inhabitants might have become Romans, but this didn't mean the end to non-Romans being part of the Roman army.
In their desperation the embattled emperors of the third century had recruited any military forces which came to hand. Germans Sarmatians, Arabs, Armenians, Persians, Moors; all were not subjects of the empire and now stood to the Roman army in the same relation as once the auxiliaries had done.
These new barbarian imperial forces might have grown larger as the third century went on, but their numbers did not pose a threat to the legions of the empire.
Ever from emperor Gallienus onward the tendency of increasing the proportion of cavalry and light infantry and relying less on the heavy infantry legionary grew more apparent.
The legions gradually were ceasing to be the preferred imperial troops.

Emperor Diocletian was largely responsible for the reforms of the army which followed the tumultuous third century.
He addressed the chief weakness of the Roman defence system by creating a central reserve. Had large invasions of barbarians broken through the defences, there never had been anyone in the interior of the empire to stop them, due to the system introduced by Augustus by which all the legions were based at the edges of the empire.
So Diocletian created a central reserve, the comitatenses, who now enjoyed the highest status among the army. They were what the legionaries in their bases along the border, now referred to as the limitanei, had once been.
These new, mobile units were organized into legions of one thousand men, rather than the traditional full-scale size of the old legion.

With the fourth century the shift toward cavalry and away from heavy infantry continued.
The old legionary cavalry completely disappeared in the face of the emerging heavier, largely Germanic cavalry.
And yet throughout the reign of Constantine the Great the infantry still remained the main arm of the Roman army.
Though the rise of the cavalry was manifested in the fact that Constantine abolished the post of praetorian prefect and instead created two positions; Master of Foot (magister peditum) and Master of Horse (magister equitum). Though still the legions held dominance in the empire. Emperor Julian still defeated the Germans at the Rhine with his legionaries in AD 357.
But the cavalry was nevertheless rising in importance. For this rise there were mainly two reasons. Many barbarians resorted simply to raiding for plunder rather than actual invasion. To reach such raiding parties before they retired out of Roman territory, infantry was simply not fast enough.
The other reason was that the superiority of the Roman legion over its opponents was no longer as clear as it had been in the past. The barbarians had been learning much about their Roman foes in past centuries. Thousands of Germans had served as mercenaries and taken their experience of Roman warfare back home with them. With this increased competition, the Roman army found itself forced to adapt new techniques and provide strong cavalry support for its embattled infantry.
If the Roman army had throughout most of the third and fourth century been undergoing a transition, gradually increasing the number of cavalry, then the end of this period of gradual change was brought about by a dreadful disaster.
In AD 378 the Gothic cavalry annihilated the eastern army under emperor Valens at the Battle of Adrianople (Hadrianopolis).
The point had been proven that heavy cavalry could defeat heavy infantry in battle.