What event in 1204 greatly weakened the Byzantine Empire?

The event in 1204 that greatly weakened the Byzantine Empire was the Fourth Crusade. Instead of reaching the Muslim-controlled Holy Land, the crusading army attacked and captured Constantinople, the Byzantine capital, in April 1204.

The diversion began with financial and political disputes involving the crusaders and Venice. The crusaders first intervened in Byzantine dynastic politics, helping Alexios IV Angelos claim the throne. When promised payments and support failed to materialise, relations collapsed. The army besieged Constantinople, captured it, and subjected the city to extensive looting.

The conquerors established the Latin Empire in Constantinople and divided much of Byzantine territory among Latin rulers and Venetian interests. Byzantine resistance continued through successor states, especially the Empire of Nicaea, the Despotate of Epirus, and the Empire of Trebizond. Nicaean forces eventually recaptured Constantinople in 1261, but the empire never fully recovered its former strength.

The Fourth Crusade is sometimes confused with the earlier First, Third, or later crusades because all involved expeditions connected with the Holy Land. Its seizure of Constantinople was especially damaging because it fractured Byzantine political authority, weakened its economy, and deepened hostility between Eastern Orthodox and Western Catholic Christians.