Fourth Crusade
A New Deal
The Venetians were practical businessmen, and no one of them was more pragmatic than their doge, 
Enrico Dandolo. There the Crusaders sat, unable to pay for their passage, unwilling to go home, and in the meantime running up bills with all the locals and equally unable to pay those. He could not play it too tough, however, for Pope Innocent III was already angry with how matters were proceeding and would not hesitate to place the city under interdict.
So, the Venetians offered a new arrangement to replace the old one. Venice had for some time ruled much of the Dalmatian coast, mainly as a way to secure control of the Adriatic and its shipping lanes. Recently, however, the King of Hungary had been inciting rebellion in the Dalmatian towns, offering them his protection. One town that had defected was Zara, which for fifteen years Venice had been trying to recover.
The doge offered to delay the payment of the contract (cancelling it was out of the question). In return, the Crusaders would help Venice recover Zara. The Crusade leaders had little choice, since the alternative was to abandon the Crusade, violate their crusading vow, and return home broke and humiliated. Even so, many in the army objected vigorously, and some even refused to go. But the Doge himself took the cross, and many Venetians followed his example.
Some time around now, a fortuitous concidence happened. Isaac II Angelus was blind and in prison in Constantinople, but his son Alexius IV had managed recently to escape and flee to the West. Early in 1202, as the Crusaders were preparing at last to depart (to attack Zara), young Alexius was in Italy and appealed to the Crusaders to help him drive out the usurper Alexius III and to him (the prince) on the throne. If they should do so, the young prince promised an extravagant amount of help for the Crusade—men, money, weapons, ships.
This appeal fits so neatly with the agenda of the principal leaders of the Crusade that many historians have smelled a plot. We won't enter here into that controversy. Whether through chance or through careful planning, it so happened that Boniface of Montferrat would be glad to participate because he might recover Thessalonica; and Venice would be glad because the prince promised to restore all their old privileges and more besides; and the rest of the Crusaders could look forward to that great pooling of resources of East and West that had been repeatedly touted in crusading thought.
So the agenda was set before the fleet ever sailed on October 1, 1202. The Crusaders would capture Zara for Venice, then would capture Constantinople for the young prince Alexius, and then would proceed on to Outremer. By this time, it was not at all clear whether the ultimate objective was still Egypt, for most of the leaders were no longer thinking much past Constantinople.