It will be a complete surprise for you when you read a bit of history and discover that the Byzantine Empire did a damn fine job of "lasting the onslaught" of the Arabs. It didn't do as good a job of resisting the Ottoman Turks, who were a completely different people. They also subdued the Arabs.
If Byzantium had not resisted the Arabs Europe would probably have been conquered and Christianity would be a minority sect. It is unlikely that relatively primitive Europe could have repulsed an attack from the east as well as one from Spain.
