As their respective commanders secured Syria, Iraq and Egypt in the 640s, forays were made into the wealthy Iranian centres of Sistan and Khurasan and into the Roman Mediterranean, Asia Minor and North Africa (see Map 4.1). In part because of the marine and mountain barriers that protected it, Roman Constantinople escaped the fate of Sasanian Ctesiphon. Whereas the Sasanian king was killed in 651, and the surviving royal family were driven across the Oxus river to seek refuge with the Hephthalites in Central Asia, geography favoured the Romans. Both the distance to Constantinople and the shelter of the Mediterranean Sea and Anatolian highlands gave the Romans, led by the young Constans II (r. 641–c. 668/9), time to organise an effective defence of what remained of Roman territory and fiscal resources.

The Romans were also saved by the deep divisions within the Arabian armies. Like most conquerors, the Arabian Faithful saw themselves as right-fully separate from the defeated, tax-paying peoples. This distinction was expressed in the Arabian Faithful's sense of themselves as especially favoured by God and their sense of their cultural difference, in customs and language, as well as in their specific economic rights to the wealth of the conquered lands. However, this sense of difference, whether expressed in religious, cultural or fiscal terms, did not in itself confer political unity. The Arabian conquerors, the migrants who followed behind them, and the pastoralists of the Syrian Desert who had joined them, often found themselves competing with one another for resources and power. In this competition, both existing kinship connections and new associations brought about by the conquests and migrations produced shifting factional interests.

Furthermore, the Arabians’ Hijazi leaders were not themselves united, and so divisions in the armies aligned with conflicts among their leaders over control of the empire and its resources. Among the Hijazis, contention focused on whom among the Meccan grouping of Quraysh should lead the empire. Between 644 and 692 there were two particularly violent episodes. The first, in 656–61, was fought between men who were younger contemporaries of the Prophet Muhammad himself; the second, in 680–92 (also often given as 683–92) was fought between their sons and other close relatives.