What is the Mandate of Heaven? If it's a document what was written in it?

The Mandate of Heaven (Tianming), also known as Heaven's Mandate, was the divine source of authority and the right to rule of China's early kings and emperors.

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).