In the meantime, control of the newly formed crusade army was taken over by the powerful Venetian doge Enrico Dandolo, who, to the surprise of all, including the pope himself, in the Fourth Crusade first sent an attack on Hungarian Zara in 1202.

The siege of Zara was the first major action of the Fourth Crusade and the first attack against a Catholic city by Catholic crusaders. The crusaders had an agreement with Venice for transport across the sea, but the price far exceeded what they were able to pay. Venice set the condition that the crusaders help them capture Zadar (or Zara), a constant battleground between Venice on one side and Croatia and Hungary on the other, whose king, Emeric, pledged himself to join the Crusade.

Although some of the crusaders refused to take part in the siege, the attack on Zadar began in November 1202 despite letters from Pope Innocent III forbidding such an action and threatening excommunication. Zadar fell on 24 November and the Venetians and the crusaders sacked the city. After wintering in Zadar, the Fourth Crusade continued its campaign, which led to the siege of Constantinople.