In a hundred years the caliphate ran from the Atlantic to the Indus. Religious enthusiasm alone does not explain that, and the chapter names the other reasons. Set each condition and watch two different clocks: how fast the armies move, and how slowly the conquered actually convert. They are not the same clock, and the gap is the whole point.
The conditions632 onward
Load a scenario, then move anything you like.
The umma cut across the tribal blood ties that had organized Arabian life. That is why it was revolutionary, why it was resisted — and why an army raised from rival clans did not simply dissolve.
Rome and Persia had just fought each other from 602 to 628. Heraclius won at Nineveh and neither state had a reserve left to field.
Recurring waves of Yersinia pestis after 541 hit the dense, connected Mediterranean world hardest and largely spared the interior of Arabia.
Heavy imperial armies could not follow a force that withdrew into terrain they could not supply themselves in. Garrison cities went up at Kufa, Basra and Fustat, on the desert edge.
At the left: convert or die, new officials, old elites destroyed. At the right: keep your religion and your tax collectors, pay the jizya as a protected dhimmi subject — frequently a lighter bill than Constantinople sent, especially for Miaphysite Christians it had been persecuting.
Real, and not sufficient. Push it to the top with everything else at the bottom and watch what happens — that is the explanation most textbooks used to give.
What your caliphate does
How far the armies get
Two clocks, 630 to 1100
land under caliphal ruleshare of subjects who are Muslim