The Rise of Islam and the Caliphates

Why this matters

Byzantine and Sasanian, two empires exhausted from fighting each other, explain this expansion's speed better than religious zeal alone: neither had the strength left to defend its own frontier once the conquests arrived. A story that credits faith alone skips the state of everyone else on the map.

Dhimmi status and a jizya tax, not forced conversion, is the specific arrangement al-Baladhuri's account of a negotiated surrender actually shows. The succession dispute that grew into the Sunni-Shia split gets the same even-handed treatment, described without favoring either side.

From one teacher to another

I won't reduce this expansion to one cause, because both of the easy stories, pure religious conquest and barely a conquest at all, fall apart fast once students dig in. Mapping the Rashidun conquests apart from the Umayyad reach kills the second myth, and working out why dhimmis paid a tax instead of facing forced conversion kills the first, before anyone reads al-Baladhuri's own account of a city surrendering. The succession dispute that becomes the Sunni-Shia split gets the same neutral treatment as everything else here.