The least successful crusade is widely considered to be the Fourth Crusade, which took place from 1202 to 1204. Initially aimed at recapturing Jerusalem from Muslim control, the crusaders became embroiled in a series of political manipulations and conflicts that led them far off course.
Instead of reaching the Holy Land, the crusaders diverted their efforts to Constantinople, the capital of the Byzantine Empire. Influenced by Venetian merchants and political ambition, they ended up sacking the city in 1204. This not only failed to achieve the original goal of the crusade but also significantly weakened the Byzantine Empire and deepened the schism between the Eastern Orthodox and Catholic Churches.
The fallout from the Fourth Crusade was profound, as it led to the establishment of the Latin Empire in Constantinople and contributed to a long period of instability in the region. In essence, what was intended to be a religious mission turned into a catastrophic event for both the Eastern and Western Christian worlds.