Eastern Roman army

The Eastern Roman army was the army of the eastern half of the Roman Empire, from the definitive division of the empire in 395 AD until its reorganisation by themes after the permanent loss of Syria, Palestine and Egypt to the Arabs in the seventh century. It continued the Late Roman army of the fourth century and at first kept the same basic organisation as its western counterpart, but between the fifth and seventh centuries it changed greatly: cavalry became more important, mobile field armies took a larger role and the border armies turned into local militias. Under Justinian I (527 to 565) much of it fought in the partial reconquest of North Africa from the Vandals, most of Italy from the Ostrogoths and parts of southern Spain, although the Plague of Justinian sharply cut its strength. In the seventh century it fought a long and destructive war with the Sasanian Empire under Heraclius, then lost Egypt and Syria to the Rashidun Caliphate after the defeat at Yarmuk, which led to the thematic system of the later Byzantine army.