During the Byzantine-Arab Wars, the Arabs (as the Umayyad Caliphate) twice attempted to siege Constantinople. Both times, they were beaten off, allowing the Byzantines to survive another 700 years. What if the Byzantines lost? What would the Umayyads do, and what effect would this have on the rest of the world? Without the Byzantine empire, there would be relatively little to stop the Muslim forces from advancing into Europe. However, due to size concerns (the Umayyads were already one of the largest empires in history), I doubt that they would go on to become a super-Roman empire. Instead, I think they might limit themselves to dominating the Mediterranean. Bad news for the Pope, but Northern France, Germany, Poland, and Russia will escape unharmed. Obviously, this will have severe butterflies. First of all, there will be A LOT more trade between (southern) Europe and the East. Importation of a lot of new ideas in agriculture and the like seems inevitable, as would increased importations of valuable Eastern goods. This might reduce trade links between Scandinavia and the Mediterranean areas, which I would expect to increase trade/raiding against surrounding areas, and possibly colonization of Iceland, Greenland, and Vinland. Viking America? The fast spread of Muslim ideas into the region might also lead to rapid advancement as Greek and Roman works are transmitted. An early Renaissance-type outburst, perhaps? I would also expect that the increased Muslim influence in the area would severely affect surrounding states. The Russians might convert to Islam this time round, since the main base of Orthodox Christianity is gone and they're right on those Baltic-Med trade routes. Overall, I think that Islam would be much more dominant in Europe right now, particularly in the more southern regions, though Christianity would remain important, I'm sure, for some time. (It doesn't seem that I can put in paragraph breaks. For some reason, it keeps reverting the ones I put in every time I use preview. What's up?)