Between about 700 and 1000 CE, the religion of Islam spread through a combination of trade, politics, and military conflict. An obligation to spread the word included domination by force of polities that were unwilling to accept Muhammad’s teachings. Strategy was an important component of the apparently rapid spread of combined religious influence and political control. By identifying areas that were susceptible to takeover, such as the fringes of the over-extended Byzantine and Persian Empires, the military leaders could destabilize the existing polities and then establish bases from which to extend their campaigns. One key aspect of military action was the extensive use of cavalry. Breeding horses and training specialized mounted warriors went hand in hand.
Recruiting converts into the holy wars was another essential step in building the armed strength for further expansion. Many subjects of the Byzantine Empire, for example, were members of different ethnic groups who lacked strong loyalty to the distant emperors. Regarding participants for their military prowess, as well as establishing governance systems that at least superficially included people of different faiths, including Christians and Jews, helped to discourage rebellion.
Regarding the question of what “most people think,” historical attitudes about the rapid spread of Islam have long varied in their respective emphasis on the peaceful versus the military aspects. In recent years, there has been increased attention to the warfare side in comparison to trade and politics. The increased availability of formerly unknown documents has also allowed scholars to re-evaluate earlier positions.