"The Bedouin had traditionally lived off raiding neighbouring tribes and extracting payment in various forms from settled peoples. It was a fundamental principle of early Islam, however, that Muslims should not attack each other: the umma was like a large and expanding tribe in the sense that all men were members of the same defensive group. If all the Arabs were now part of one big family, raiding each other was clearly out of the question. The inhabitants of the settled communities were also fellow Muslims. A peaceful, Muslim Arabia would mean abandoning both of the traditional nomad ways of surviving. The alternatives were stark: either the Islamic elite were to lead the Bedouin against the world beyond Arabia and the desert margin, or the Islamic polity would simply disintegrate into the warring constituent parts and the normal rivalries and anarchy of desert life would reassert themselves once more... The only way of avoiding an implosion was to direct the Muslims against the non-Muslim world."
"A key element in the success of the conquests was the comparatively easy terms usually imposed on the conquered. Arab commanders were normally content to make agreements that protected the lives and properties of the conquered, including rights to their places of worship, in exchange for the payment of tribute and the promise that they would not help the enemies of the Muslims. Defeated defenders of cities that were conquered by force were sometimes executed, but there were few examples of wholesale massacres of entire populations."
"...the Muslim conquerors put little or no pressure on the recently subjected populations to convert to Islam. Any attempt at compulsory conversion would probably have provoked widespread outrage and open hostility. As it was, the Muslim authorities established working relationships with the heads of the churches and other religious institutions that were now in their power. Conversion when it came was partly the result of fiscal pressures, the desire to escape the hated poll tax, but also because conversion provided an opportunity to escape from existing social constraints and to become part of the new ruling class."
Hugh Kennedy, The Great Arab Conquests