During the third century the Roman empire ran into severe difficulties. The origin of these problems began during the second century, during which period an epidemic occurred (possibly measles or influenza) and the empire suffered invasions by Parthians in the east and the Germans from the north.

The disease caused a large decline in the tax-paying population, which reduced the tax income paid to the State at a time the increased military expenses were needed; the result was an inability to meet the military payroll even with tax increases. Debasement of the currency caused an economic disruption and severe annoyance to the army, followed by more barbarian invasions. Internal revolt and external threats led to severe turmoil.

This led to the reforms of 284. Diocletian replaced quality with quantity, chosing to cut the pay of soldiers (being a soldier had already become a less than desirable job, while in the early empire it had been a very good job). Cutting the payroll and instituting a new form of taxation (goods instead of cash) allowed Diocletian to vastly increase the amount of soldiers available; however, it also decreased the individual soldier's motivation and morale. According to Warren Treadgold an army totalling roughly 390,000 soldiers in 285 was increased by half again to 581,000.

Constantine removed the majority of the fighting troops far behind the frontiers, leaving these to be guarded by troops of inferior quality.

This same period during the third century saw a transition from an infantry focused to a cvalary focused army. There are several reasons for this.

The primary reason was most likely the changed circumstances with respect to the enemies of the empire. The Romans throughout history have always adapted the weapons of their foes to their own service. The spanish sword or the celtic armor and helm are evidence of this. In the early empire the primary opponents for the legions was still the celts of spain and britain and the germans. These groups remained predominantly infantry armies, although they did have cavalry elements. The Persians were also a mostly cavalry army, but during the Principate there was no long-lasting open conflict between Rome and Persia.

However the second and third centuries saw the enemies of the empire change. The Celts and Germans were replaced with the Goths and Huns, who were horse warriors moving from the steppes. At the same time the Persians were replaced by the far more aggressive Parthians. With their enemies increasingly using cavalry as the focus of their armies, Rome began to do likewise.

At the same time, the reforms of Diocletian and Constantine had an effect. Most of the limitanei were locals who served more to fight bandits than barbarians. Although in a large war they could be mobilized, they were nowhere near the quality of the old legionaries and by being infantry they were cheaper to equip. The mobile and permanent field armies (comitatenses) were cavalry.

This was for several reasons. Because field armies were located far behind the frontiers they had the ability to move to oppose any barbarian invasion but this also required they had the ability to move rapidly, and cavalry can do so faster than infantry. Another reason was that the field armies, being the main defense for the empire, got the majority of the money and training, and since they were the best quality troops providing them with horses (which require extensive experience for maximum efficiency) makes sense. Finally, much of the comitatenses was in fact composed of barbarians, mostly under their own leaders, and since the barbarians were horse-warriors at home they remained so in Roman service.