As we saw in Chapter 7, continuity from Roman imperial times was attenuated in the western half of the empire by economic decline, political division under barbarian kings, and social dislocation. Initially, in contrast, the eastern half of the empire retained a more vital economy and urban social organization and, above, all maintained political unity centered on Roman imperial ideology and mechanisms of rule. The military organization of the state historians often refer to as the Eastern Roman Empire therefore also showed greater continuity with late Roman military institutions. Indeed, many units of the Roman army could trace their history back several centuries in 350 and would continue to be able to until the 1070s.
But transformation came to this part of the Roman world just as it did in the west. Long-term economic and demographic developments played their part as they did in the Germanic kingdoms, but the crucial period of transformation was the first half of the seventh century. First, East Rome entered into a protracted life-or-death struggle with the Persian Empire, revived under the native Persian Sassanid dynasty, over control of the provinces of Syria and Egypt. Then, just as those wars had been settled in Constantinople’s favor, Arabs united under a new religion burst out of Arabia. Persia was swallowed whole, and the richest Roman provinces again fell to conquest, this time for good. The truncated and reorganized empire that survived to face another century and a half of life-or-death struggle against the Caliphate is what historians usually call Byzantium. Though still an heir to Rome, its connections to the past were more distant, and its original features more prominent.
The Islamic world that emerged from the Arab conquests was also an heir of Rome and an element of transformation. Like the west and Byzantium, much Islamic territory saw patterns of warfare shaped by surviving Roman infrastructure—roads, city walls, and even administrative structures—while Islamic culture absorbed a large measure of Roman and Hellenistic science. On the other hand, outside of the organization of its naval forces (see Chapter 10), Islam was in some ways the farthest from Rome politically and militarily. In this chapter, we examine these developments in detail.