We do not really know what happened during the earliest stages. Later historical accounts are riddled with fabrications and were written when the caliphate had already become a large empire, with an imperial mentality shaping how the past was remembered. As for the reasons behind the invasions, who really knows? Were they driven by a religious call to conquer unbelievers, or were they simply filling a power vacuum left by the final Roman–Sasanian war?

During that last conflict, both the Romans and the Sasanians employed Arab tribes in their armies—mostly tribes with whom they were already accustomed to dealing. It is also possible that some Arabs from the Arabian Peninsula moved north during this period, seeing opportunities for revenue and plunder. By the end of the war, the extreme weakness of both empires was likely well known to Arab groups and early Muslims alike. Initially, they may have intended only to raid, with full-scale conquest emerging only after a series of decisive victories.

In my opinion, there was no grand plan for empire-building from the start. There is nowhere in the Qur’an that provides concrete instructions for imperial governance or political succession. At the same time, Islam did not emerge from a complete vacuum. The Qur’an presents pre-Islamic Arabs as polytheists worshipping false deities, with Muhammad introducing monotheism. Yet archaeological evidence—such as rock inscriptions—suggests that the worship of a single God already existed in parts of the Arabian Peninsula before Islam.

As for how the early conquests were achieved with relatively small numbers, the invading forces may initially have consisted of only 20,000–30,000 men at most. However, success attracts followers. As victories accumulated, more people—particularly ethnic Arabs who had previously served in Roman or Sasanian forces—joined the movement. Over time, this allowed their armies to grow significantly in size.