Some attribute the early and rapid spread of Islam to forced conversions by the sword. While it is accurate that the Muslim empire initially spread, for the most part, through battles and conquests (a common phenomenon for that time) the religion of Islam itself was never forced on anyone who found themselves living under Muslim rule. In fact, non-Muslims were afforded the right to worship as they pleased as long as a tax, called "jizyah", was paid. During the Dark Ages, Jews, Christians and others were given protection by the Muslims from religious persecutions happening in Europe. Islam teaches no compulsion in religion (Qur'an 2:256 and 10:99). For more, read "The Spread of Islam in the World" by Thomas Arnold.