Heraclius wins at the Yarmouk...

[regertz]

The Byzantines under Emperor Heraclius instead of his brother win at the Yarmouk River and drive the new forces of Islam back into Arabia, saving the eastern provinces.  Very likely the crumbling Persians who had already negotiated with Heraclius for a combined attack on the Muslim forces would have rallied a bit and perhaps saved their empire as well or at least some portion of it.  Discouragement at the disasters might have tempered faith for the new Muslim religion and caused a quick collapse, though I suspect the end result would have been a border war similar to the Persian-Roman standoff. between Byzantium and Arabia with possibly the Persians returning to major power status a few decades later.  North Africa remains Roman and emperors are able to devote some attention to the troubles in the Balkans, possibly even Italy.  Eventually Europe is divided with Byzantium holding part or all of Spain and Italy and regaining the Balkans long before the Macedonian dynasty achieved reconquest in our timeline.  Arabia is soon torn by faction and Persia regains her status though with perhaps a bit more friendly relations toward her Byzantine savior, perhaps a grudging but strong British-French relationship developing over time with mutual aid responses to the challenges of the Mongols and Turks.

[Dan the Roman]

Byzantium was in trouble anyway. Herecleus was already beginning to ail, and when he died, he was going to leave a family situation which was going to explode. Probably not as disastrously for the Empire as it did. And the Arabs would not have been gone for good. They lost battles against the Persians prior to Qadisaya, and they might well have returned. But there would have been no dramatic Arab conquest, and Umayed takeover in Medina might have rpevneted one altogether.