Fourth Crusade
Pope Innocent III declared the Fourth Crusade with the intention of capturing Jerusalem from Muslim rule by invading through Egypt. He was able to organize a crusader army, but they were unable to get to the Holy Land. Following their conquest of the Adriatic city of Zara, the Crusaders joined the conflict for the Byzantine throne.
The Fourth Crusade didn't achieve its goal; instead, it resulted in the sack of Constantinople, the greatest Christian city and the capital of the Byzantine Empire, in 1204. The city's treasures, artifacts, and relics were looted, and the Byzantine Empire as a whole was split between Venice and its allies.
One of the last events in the Great Schism between the Eastern Orthodox Church and the Roman Catholic Church was this. A crucial turning point in the decline of the Byzantine Empire and, with it, of Christianity in the Near East, was the siege of Constantinople. The Fourth Crusade earned its notorious reputation as the most unscrupulous and profit-driven of all the crusades.
Time: 1202 - 1204