What impact did the caliphs have on the spread of islam

The caliphs used the authority that Islam gave them to spread Islam by war and conquest across a large part of the world.

the conditions that made the byzantine empire vulnerable to attacks by the muslin turks were the sack of constantinople by western crusaders, the conquests of the sultanate of rum, presence of rival kingdoms and finally the set up of fiefdoms at the empire which were owned by the ghazis (turkish warriors).