Islam's Rapid Expansion

Why was Islam able to expand so rapidly early on?

The mercantile system in the region is a good reason. The Middle East is the MIDDLE for a reason, which is that many merchants had to travel across it to sell wares to the Persians and Byzantines on each side of the Arabs. Beyond this, in order to make deals with the Muslims, many non-Muslims felt compelled to convert so as to make the transactions smoother.

But there is more to conversion than this. The early Caliphs were smart enough to realize that the unified tribes, who were once enemies, needed their battle hunger channeled into a new direction: the northern settled lands and the empires on either side of them. From what I have read so far, the Middle East is the true birthplace of guerrilla warfare, and many of the Muslim armies were good at it.

Also helpful to the Muslim cause were two instances of discontent with the Byzantines and Persians. The first comes from Arab mercenaries that fought for either side; the Byzantines became unable to pay, and the Persians felt they were too rich to need mercenaries, so the Arab tribes quit and eventually converted. Added to this are the Copts and Jacobites, who were essentially heretics to the Orthodox Christians -- these people were tired of their status and allowed the Muslims to come in and take control of their homelands.

Not to be neglected is the fall of the Sasanid Empire, as well. This collapse allowed the Muslims to come into formerly Persian-controlled regions and take control.

Outside of these reasons, there were some forced conversions and the Caliphs' governments were great at bringing unity, but these factors seem to have less to do with spreading Islam than the previously mentioned causes.