Eastern Roman army

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The Eastern Roman army refers to the army of the eastern section of the Roman Empire, from the empire's definitive split in 395 AD to the army's reorganization by themes after the permanent loss of Syria, Palestine and Egypt to the Arabs in the 7th century during the Byzantine-Arab Wars. The East Roman army was the continuation of the Late Roman army of the 4th century, until it gradually transformed into what is now called the Byzantine army from the 7th century onwards.

The East Roman army was a direct continuation of the eastern portion of the late Roman army, from before the division of the empire. The East Roman army started with the same basic organization as the late Roman army and its West Roman counterpart, but between the 5th and 7th centuries, the cavalry became more important, the field armies took on more tasks, and the border armies were transformed into local militias.

In the 6th century, Emperor Justinian I, (r. 527–565 AD), sent much of the East Roman army to try to reconquer the former Western Roman Empire. In these wars, the Eastern Roman Empire reconquered parts of North Africa from the Vandal Kingdom and Italy from the Ostrogothic Kingdom, as well as parts of southern Spain. The power of the army diminished in his reign owing to the Plague of Justinian. In the 7th century, Emperor Heraclius led the East Roman army against the Sasanian Empire, temporarily regaining Egypt and Syria, and then against the Rashidun Caliphate. His generals’ defeat at the Battle of Yarmuk would lead to the Islamic conquest of Syria and Egypt, and would force the reorganization of the East Roman army, leading to the thematic system of later Byzantine armies.