Pope Innocent III issued the call for a new crusade, aiming to restore Latin control in the eastern Mediterranean and ultimately recover Jerusalem—setting in motion the expedition later diverted to Zara and Constantinople.
Crusade leaders contracted with Venice for maritime transport in exchange for 85,000 marks. The later inability to pay this sum became a key driver of the crusade’s redirection toward targets beneficial to Venice.
The main crusader army gathered at Venice in spring 1202 in numbers smaller than expected and could not meet its financial obligation, leaving the expedition vulnerable to Venetian leverage over its route and targets.
The crusader-Venetian fleet sailed from Venice in early October 1202, beginning the expedition that soon turned against Zara rather than proceeding directly toward the intended eastern campaign.
In response to the assault on a Christian city, Pope Innocent III excommunicated the crusading army en masse—highlighting the breach with papal intent and the growing legitimacy crisis of the expedition.
Crusaders and Venetians besieged Zara (Zadar), a Christian city under the Hungarian crown. The city surrendered after roughly two weeks, and the crusading force wintered there—an early, controversial diversion from fighting Muslim powers.
While wintering at Zara, the crusade leadership committed to sailing to Constantinople the following spring, tying their prospects to Byzantine dynastic politics and promised compensation rather than the original Holy Land objective.
The crusader-Venetian force besieged Constantinople in summer 1203, pressuring Emperor Alexios III. The fighting culminated in regime change and placed the crusaders in a decisive position to influence imperial succession.
After Alexios III fled, the Byzantines restored the blinded Isaac II and elevated his son Alexios IV. Alexios IV attempted to fulfill promises to the crusaders but faced fiscal collapse and intense anti-Latin hostility in the capital.
A palace coup toppled Alexios IV; Alexios V Doukas took the throne, and Alexios IV was killed soon after. The new regime rejected crusader demands, pushing the western army toward open war for control of the city.
Crusader and Venetian leaders concluded a preliminary agreement for post-conquest rule: a 12-member electorate (6 Venetians, 6 crusaders) would choose a new emperor, and territories—including Constantinople—would be apportioned among victors.
The crusaders launched an initial large-scale assault on Constantinople in early April 1204, but the defenders repulsed it, foreshadowing the more decisive second attack days later.
On 12 April 1204, the crusaders and Venetians successfully broke into Constantinople (notably via attacks on the sea walls along the Golden Horn). The capture initiated three days of widespread pillage and destruction in the city.
From 12–15 April 1204, crusader forces looted Constantinople on a massive scale, including the desecration and theft of church treasures—an event that deeply damaged Byzantine power and widened Latin–Greek hostility.
Following the conquest, the agreed mixed Venetian-crusader body elected Baldwin of Flanders as ruler of the new Latin polity, formalizing the political outcome of the crusade’s diversion.
Baldwin was crowned in Hagia Sophia on 16 May 1204, marking the institutional start of the Latin Empire in Constantinople and the displacement of Byzantine imperial authority from the capital.
Fourth Crusade and the Sack of Constantinople (1202–1204)
