In 614, the Persians conquered Jerusalem, the most holy city in Christendom. It is hard to overestimate how alarming this was to the Christians of the Roman Empire. "The True Cross on which Jesus was crucified was captured and sent back to the Persian capital as a trophy of war."
The Byzantine Empire (what was left of the Roman empire) won back Jerusalem in 627, seriously weakening the Persian Empire in the process. By that point, the Persian and Byzantine empires had both been decimated. Between 628 and 632, the Persian Empire dramatically collapsed and anarchy took its place through much of its old empire. It was into this milieu that Islam emerged.
In 610, Muhammad began to receive revelations. In 622, he fled to Medina, a date that would become year one in the Islamic calendar. His revelations came out of a time when the old empires were crumbling and the holiest city was under hostile occupation.
How Islam Spread:
As the Persian Empire collapsed, Muslims began to conquer the lands and cities they lost.
"Willing to sanction material gain in return for loyalty and obedience, Muhammad declared that goods seized from non-believers were to be kept by the faithful. This closely aligned economic and religious interests.
"Those who converted to Islam early were rewarded with a proportionately greater share of the prizes, in what was effectively a pyramid system. This was formalized in the early 630s with the creation of diwan, a formal office to oversee the distribution of booty. A share of 20 percent was to be presented to the leader of the faithful, the Caliph, but the bulk was to be shared by his supporters and those who participated in successful attacks. Early adopters benefited most from new conquests while new believers were keen to enjoy the fruits of success. The result was a highly efficient motor to drive expansion."
|The city of Baghdad from about the 10th century|
Given the Muslims were filling in a vacuum left by falling empires, conquest sometimes required little in the way of battle. "Damascus, for instance, surrendered quickly after terms were agreed between the local bishop and attacking commander." Basically, the folks in Damascus could keep their churches open but were now expected to pay tax to the prophet rather than Constantinople. Later, the Muslims' conquest of Egypt tripled their income from taxes and often just the threat of military force against other people was enough to provoke negotiation and surrender. The Muslims ignored Europe because it was so poor, concentrating instead on the Middle East and eastward to the border of China. The result was worth billions of dollars (in today's terms), making the Muslims and all of those conquering converts rich. Very rich. One wedding in what is now Baghdad included presents from the groom to people all over the country: "gold bowls filled with silver and silver bowls filled with gold were taken around and shared out ..."
In the wake of these conquests, the Muslim world was incredibly wealthy. They were not only materially rich but intellectually rich, with leading thinkers in philosophy, physics and geography. Their thought leaders wrote about medicine and lovesickness, how the world revolves around the sun, and the concept of zero. Money funds leisure and even the pursuit of knowledge. Not all of these great thinkers were Muslim but they were drawn to its world and resources.
Islam is a religion. Curiously, its expansion seems to have been fueled by a very clever business model: profit sharing from conquest for anyone who converted. It was a model that seemed, in retrospect, to ensure plenty of converts and plenty of cities and territories in which they could live.