With Islam in the news recently, its worth remembering that Arabia for a time was united under a theocratic Islamic state called the "Caliphate" that actually defeated the Persians and took over Mesopotamia and also took over much of Syria, before being swatted away by Heraclios and the East Romans in the little known battle of Yarmouk.
The obvious alternative history question is what would have happened if the Caliphate had survived and defeated the East Romans at least as badly as they defeated the Persians?
The obvious alternative history question is what would have happened if the Caliphate had survived and defeated the East Romans at least as badly as they defeated the Persians?