This period covers Septimus Severus’ reforms of the Roman army at the end of the 2nd century AD to the fall of the Western Roman Empire in the 5th century AD. The surviving Eastern Roman Empire continued until the fall of Constantinople in 1453. In addition to a long series of civil wars and internal conflicts, Rome's greatest rivals were the Sassanid Persians who overthrew their Parthian overlords in the 3rd century AD. Warfare between the two empires was almost continuous for the next 400 years which led to the eventual exhaustion of both protagonists and the destruction of the Sassanids by the Islamic Arabs in 651 AD.
After many years as the pre-eminent imperial power, the Roman Empire by the 3rd Century AD had reformed the army into a leaner fighting machine, based around mobile field armies, a defense in depth and border troops made of older, settled, garrisons. More emphasis was placed on the cavalry, with the adaptation of horse archers, more flexible cavalry and super-heavy cataphracts, ideas borrowed from its Persian opponents. The infantry still had its legions and auxilia, but more emphasis was placed on archers and mercenaries to bolster the lines.
Succeeding the Parthian Empire, the Sassanian dynasty ruled Persia from 224 until its conquest by the Arabs in 651 AD. Many wars were fought with Rome/Byzantium for control of the Middle East, peaking in 621 with the conquest of Egypt.