Fourth Crusade and the Sack of Constantinople
Bound for Egypt but in debt to Venice, the crusade turned aside, captured and sacked the Christian city of Constantinople, and set up the Latin Empire.

Eugène Delacroix · Public domain · Source
Pope Innocent III called the crusade, but when the knights could not pay Venice for their fleet, Doge Enrico Dandolo had them attack Zara (today Zadar, Croatia), a Christian city of the King of Hungary, instead. A Byzantine exile prince then promised money and troops, and the crusade sailed for Constantinople.
When the promises went unpaid, the crusaders stormed the city on 13 April 1204 and sacked it for three days; the four bronze horses of St Mark’s were carried off to Venice. They founded the Latin Empire and Venice took islands and ports. The Byzantines held out at Nicaea and regained the city in 1261 but never recovered their old strength, and the rift between the Eastern and Western churches deepened.