Fourth Crusade
A Conflict of Emperors
The Comnenus family was ruling Byzantium in 1095, when the First Crusade began, and they were still ruling ninety years later. The last of the Comneni was 
Andronicus Comnenus, a fellow with a most remarkable history behind him before he ever ascended the throne, in 1182. He ruled with a heavy hand, was widely hated, and when the provinces rebelled, the people of Constantinople rioted and killed Andronicus in 1185. He was succeeded by Isaac II Angelus.
Isaac had his hands full. Bulgaria rebelled successfully, as did Serbia. A few years later, Frederick Hohenstaufen marched through his lands with a huge army; Isaac was unable to prevent the Emperor from temporarily capturing both Adrianople and Philippopolis. In the end, Frederick continued on, but only after Isaac pretty much granted him everything he wanted. He managed to recover a bit of lost ground in the ensuing years, but at the same time he lost control within the palace itself. In April of 1195, his brother Alexius III usurped the throne and had Isaac blinded.
Alexius III Angelus was no more effective than his brother and was rather more corrupt. When Emperor Henry VI pressured him, he levied a special tax to buy him off. When the Bulgars rebelled again, Alexius was unable to control them. When Serbia threw its allegiance to Hungary, Alexius could do nothing. And all the while, he continued to drain the treasury. And when the Fourth Crusade drew near, he put up a token resistance, then grabbed all the money he could find and ran.