The armies of the Fourth Crusade capture Constantinople again and establish the Latin Empire of Byzantium, but not before they sack the city and rape its inhabitants for three straight days — and during Easter week, too.
Alexios V Ducas is forced to flee to Thrace. His predecessor, Alexios IV, was the reason why the Crusaders were there to sack the city in the first place.
He made a deal with them in 1203 to put him on the throne in exchange for massive financial and military support for the Crusades. When he took power, though, he found that there wasn't enough money to keep his promises and now the Crusaders are tired of waiting.
Pope Innocent III protests at the behavior of the Crusaders, but he does not hesitate to accept a formal reunion of the Greek and Latin churches.