Despite threats of excommunication from the pope, Crusaders attack the Christian city of Constantinople.
The instigator of this was Alexios IV Angelos, an exiled Byzantine prince who in 1203 cut a deal with Crusade leader Boniface of Montferrat: get rid of his brother Emperor Alexios III and put him on the throne in exchange for massive Byzantine support for the Crusades.
What's more, Alexios IV offered to put the Byzantine Christian church under the authority of the pope in Rome. Emperor Alexios III was completely unprepared for this unexpected assault from fellow Christians and he fled. Alexios IV quickly moved in to take over, but he found that there isn't enough money to keep all of his promises to the Crusaders.
Over the past year the Crusaders have grown tired of waiting and decide to take control of the city themselves. This first attack fails, but a second attack in a few days succeeds.