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Why did the Fourth Crusade attack and sack Constantinople in 1204?

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Hagia Sophia: mosaic — Byzantine Empire The Virgin Mary holding the Christ Child (centre), Justinian (left) holding a model of the Hagia Sophia, and Constantine (right) holding a model of the city of Constantinople; mosaic from the Hagia Sophia, 9th century.

The Fourth Crusade’s attack and sack of Constantinople in 1204 was a betrayal of its original mission, driven by financial desperation and political opportunism rather than religious intent.1

The diversion began when the Crusader army, which was significantly smaller than projected, failed to pay the Venetians for the fleet and provisions they had contracted. To settle this debt, the Crusaders agreed to assist the Venetian Doge, Enrico Dandolo, by capturing the Christian city of Zadar.231

While at Zadar, the Crusaders were approached by the exiled Byzantine prince Alexius, who promised them vast sums of money, military support, and the submission of the Byzantine church to Rome if they helped him reclaim the throne. The Crusaders accepted this offer and installed him as Alexius IV in 1203. However, the new emperor soon ran out of funds and faced intense anti-Latin hostility within the city. After he was toppled in a palace coup in 1204, the Crusaders—cheated of their promised reward and disgusted by the perceived treachery—declared war on the city, leading to its brutal sack.21