Pope Innocent III proclaims the launch of the Fourth Crusade. Almost since the beginning of his reign he has been hoping to launch a Crusade against the Muslims who control the Holy Land; this marks his first attempt.
Crusades are always expensive, and this one is paid for with a 1/40th tax on the income of all clergy throughout Europe — a move so unpopular that the money becomes very difficult to collect.
Pope Innocent III even considers leading the Fourth Crusade himself instead of leaving it to secular leaders. Perhaps he should have, because the Fourth Crusade is a disaster for Christendom. Instead of heading directly for the Holy Land, it's diverted to Constantinople, where the Crusaders capture the city and install their own emperor.