What if the Rashidun forces had been defeated at Yarmouk? Obviously the implications depend on the kind of victory. If it's only a minor defeat for the Muslims than history probably progresses similarly to OTL. However, what would be the ramifications of a decisive Byzantine victory? At this point Muslim control of Iraq is secure, but the rest of Persia is still Sassanian. I think that even a few years of peace after Yarmouk would be enough for the Byzantines to regroup. What say you?