Question

How did the Muslim culture expand so rapidly?

Answer 1

While !!!!
1) Arabia - What's that?: Prior to Mohammed, Arabia had been seen as a cultural and political backwater barely worth notice and therefore Mohammed's consolidation of the Arab Tribes was not seriously considered by the Byzantine and Sassanian Empires. This lead to a very porous southern defense and miscalculation of the Islamic Armies.

2) Byzantine-Sassanian Wars: Since the Byzantines and Sassanians considered each other the only real civilizations in the region, they fought each other numerous times in the early 600s C.E. over control of what is today East Turkey and Iraq. These were intense religious wars (Christian vs. Zoroastrian) and greatly weakened both empires at just the wrong time.

3) Tribal Command Structure: Arab Tribes and warriors owed their loyalty to their Sheikhs and leaders without fault because the Sheikh provided each person with all of the amenities and protection necessary to live a full life. Whenever the Arabs went to war, the Sheikhs could count on the ability to muster their tribesmen as soldiers without issue.

4) Support of the Locals: Since the Byzantine Empire and Sassanian Empire were engaged in religious zealotry, it stands to reason that they did not tolerate their religious minorities very well. The Muslims professed to (and did) treat minorities far better than either of these Empires. This led to religious minorities (especially Jews) from within the conquered territories helping Muslims to secure outposts and positions, freeing the Muslims to continue to spread the religion.
~Jess~