Late Imperial Rome
This period covers Septimus Severuss 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, the Roman'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.