After the death of Muhammad the Muslim community split. Why and how did this occur?

The divide originated with a dispute over who should succeed the Prophet Muhammad as leader of the Islamic faith he introduced. Today, about 85 percent of the approximately 1.6 billion Muslims around the world are Sunni, while 15 percent are Shia, according to an estimate by the Council on Foreign Relations

Explanation:

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).