Thousands of crusaders, unable to pay the Venetians the full sum promised for their transport, set sail under an arrangement that diverted the crusade toward Christian rather than Muslim targets. The expedition would eventually besiege and sack Zara and then Constantinople itself, a Christian Byzantine city, in one of the era's most consequential betrayals of stated purpose.
The Fourth Crusade began with grand intentions: retake Jerusalem by first striking Egypt, the seat of Muslim power in the region. But the crusader army that gathered in Venice was far smaller than promised, and its leaders had contracted with the Venetians for a fleet sized for 33,000 men when barely a third of that number showed up.
Unable to cover the enormous debt, the crusaders struck a fateful bargain with the aging, blind Doge Enrico Dandolo: help Venice capture the rebellious Christian port of Zara in exchange for delayed payment. It was the first diversion of many.
The fleet that departed Venice would eventually be drawn into Byzantine dynastic politics, laying siege to Constantinople itself in 1204. The sack that followed—the pillaging of the greatest Christian city in the world by men who had sworn an oath to fight for Christendom—shattered the Byzantine Empire and poisoned relations between the Orthodox East and Catholic West for centuries.