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Some brief thoughts on the 'Arab conquests'. Firstly on the plague: I find it quite plausible that the period of epiicdem disease that runs roughly 540-770 removed some of the advantages of big urban concentrations: it may have made it harder to concentrate large armies and ... https://twitter.com/israr_ahmed050/status/1346143094824079361
reduced tax revenue. That said these things are only hinted at in the sources. But in this specific case, the death of Kavad II Shiroe stimulated a major succession crisis in the Sasanian empire, which was only resolved once the Arabs had already seized Ctesiphon...
One thing the plague may have done is to escalate a sense of imminent apocalypse. We see this in the frequent references to the end times in the Quran, but also in the writings of many other populations...
This brings us to religious zeal. I do not doubt that some of the conquerors will have been motivated by a keen sense of the rewards of the afterlife. The ability to seize heavily fortified sites like Dara may depend on high levels of morale...
but we should be wary of thinking that only Muslims possessed zeal or conviction. The Romans had just been victorious in a religiously charged war with the Persians. Heraclius' propaganda may well have found its way into the Quran, not least in the idea of heavenly rewards ...
for martyrs. I rather think that a series of military defeats broke Roman confidence, and that if the early Muslims had been similarly defeated then the invasions of Umar would also have crumbled. (There are reports of a Muslim army converting to Christianity in Egypt...
after being defeated). Finally, to make an obvious point, both Romans and Persians were exhausted by some twenty-five years of constant warfare. This had already led the deposition of the shah Khusrau Aparvaz and the Greek historian Theophanes reports that the Romans were...
no longer able to pay their Arab allies. These former allies may have found Muhammad and his successors to be more reliable paymasters after their impressive victories.
