How did the Fourth Crusade end up targeting Constantinople instead?

The Fourth Crusade ended up targeting Constantinople due to a diversion caused by financial difficulties and Venetian manipulation.

The Fourth Crusade, originally intended to recapture Jerusalem from Muslim control, was launched in 1202. However, it ended up sacking the Christian city of Constantinople, the capital of the Byzantine Empire, in 1204. This unexpected turn of events was primarily due to financial difficulties faced by the Crusaders and the manipulative influence of the Venetians.

The Crusaders had contracted the Venetians to provide a fleet for their journey to the Holy Land. However, they were unable to pay the agreed sum, leaving them indebted to the Venetians. The Venetians, led by Doge Enrico Dandolo, saw an opportunity to use the Crusaders to further their own interests. They proposed that the Crusaders help them capture the city of Zara, a former Venetian territory that had rebelled, as a means to settle their debt. Despite the Pope's disapproval, the Crusaders agreed, marking the first diversion from their original goal.

The second diversion came when Alexius Angelus, the son of the deposed Byzantine Emperor Isaac II, sought the Crusaders' help to reclaim his father's throne in Constantinople. In return, he promised to support the Crusade financially and militarily. The Venetians, eager to establish a favourable regime in Constantinople, encouraged this alliance. The Crusaders, still in need of funds, agreed to the proposal.

However, once in Constantinople, the Crusaders and Venetians found that Alexius could not fulfil his promises. Tensions escalated, leading to violent clashes between the Crusaders and the citizens of Constantinople. Eventually, the Crusaders and Venetians decided to sack the city to compensate for their efforts.

The sack of Constantinople marked a significant deviation from the original purpose of the Fourth Crusade. Instead of fighting against the Muslims in the Holy Land, the Crusaders ended up attacking fellow Christians in Constantinople. This event severely damaged relations between the Western and Eastern Christian churches, and the Fourth Crusade is often seen as a major factor in the eventual fall of the Byzantine Empire.