Civil wars. Probably the most important single cause of Byzantium's collapse was its recurrent debilitating civil wars. Three of the worst periods of civil war and internal infighting took place during Byzantium's decline.
Over time, its economic and military might waned and along with it, the empire's capacity to seize an opportunity. Add in civil unrest, natural disasters and powerful enemies such as the Arabs, Seljuk Turks, Bulgars, Normans, Slavs, and Ottoman Turks, and you can see why the Byzantine Empire eventually crumbled.
