...Now that you're all here and breathing fire at me, I'll explain what I mean (it's not quite as inflammatory as it sounds and no, I haven't become a Republican troll in the last month. 
)
Most "Smaller Islam Post-622" timelines involve the Byzantines, for a couple of reasons: one, the Byzantines are the shield of Europe, and protecting Christendom is the (unspoken) urge behind a lot of Islam-gank timelines here. Second, the Byzantines are by far the strongest non-Muslim state west of China, going toe-to-toe with the Caliphate(s) for the half a millennia, and, between ~850 and 1071, arguably the strongest state west of China period. Third, everyone loves the Byzantines.
But - the big moment in Islamic history, when it really took off and beat out Alexander and the Mongols for biggest, fastest, most permanent conquests, was the back-to-back battles of Yarmuk and Qadasiya in 636/7, where they defeated the Romans and Persians respectively and opened pretty much the whole Middle East to their armies. The reason - or at least a reason - why this was possible was that both empires, and their armies, were still exhausted from a two-decade war that (unlike previous Roman-Persian border wars) chewed up pretty much everything between the Nile and the Zagros.
Now, the usual (as far as I can tell) way out of this is to somehow get rid of the whole war, or at least have the Romans spectacularly more successful, leaving them better able to hold off the storm out of Arabia, after which the stronger, Egypt-and-Levant-including Roman Empire should be able to deal much better than the relatively impoverished Byzantines could. However, how about the Persians? If Heraclius gets knocked off in the chaos of the 610s and is replaced by Nikephoros Inkompetantos or whoever, could the Persians lock down the Levant fast enough and firmly enough to fight off the Arabs? Any thoughts? (It took a while for the Byzantines IOTL to accept that the Middle East was gone, but for the sake of argument assume that Khoroses II can beat them crushingly enough that the attempt at reconquest is postponed a couple of decades.)
)Most "Smaller Islam Post-622" timelines involve the Byzantines, for a couple of reasons: one, the Byzantines are the shield of Europe, and protecting Christendom is the (unspoken) urge behind a lot of Islam-gank timelines here. Second, the Byzantines are by far the strongest non-Muslim state west of China, going toe-to-toe with the Caliphate(s) for the half a millennia, and, between ~850 and 1071, arguably the strongest state west of China period. Third, everyone loves the Byzantines.
But - the big moment in Islamic history, when it really took off and beat out Alexander and the Mongols for biggest, fastest, most permanent conquests, was the back-to-back battles of Yarmuk and Qadasiya in 636/7, where they defeated the Romans and Persians respectively and opened pretty much the whole Middle East to their armies. The reason - or at least a reason - why this was possible was that both empires, and their armies, were still exhausted from a two-decade war that (unlike previous Roman-Persian border wars) chewed up pretty much everything between the Nile and the Zagros.
Now, the usual (as far as I can tell) way out of this is to somehow get rid of the whole war, or at least have the Romans spectacularly more successful, leaving them better able to hold off the storm out of Arabia, after which the stronger, Egypt-and-Levant-including Roman Empire should be able to deal much better than the relatively impoverished Byzantines could. However, how about the Persians? If Heraclius gets knocked off in the chaos of the 610s and is replaced by Nikephoros Inkompetantos or whoever, could the Persians lock down the Levant fast enough and firmly enough to fight off the Arabs? Any thoughts? (It took a while for the Byzantines IOTL to accept that the Middle East was gone, but for the sake of argument assume that Khoroses II can beat them crushingly enough that the attempt at reconquest is postponed a couple of decades.)