Crusaders arriving at the land and sea walls of Constantinople, from a Venetian manuscript (ca. 1330) if La Conquete de Constantinople by Geoffrey of Villehardouin, who took part in the Fourth Crusade. When the Venetian force's entry into the city was pushed back by the imperial bodyguard, they set fire to a number of buildings and burned a large section of an affluent suburb. It was a harbinger if worse destruction to come.
On 11th November 1202 the crusaders landed at Zara on the Adriatic and quickly made camp. The citizens saw the large army and its siege engines and knew that resistance was impossible, so they promptly sent out a delegation offering to surrender the city if their lives were spared. This was agreeable to Dandolo, who asked the delegates to remain in his tent while he went to confer with the barons.
In Dandolo's absence, Simon de Montfort the Elder (1160-1218), the leader of a small faction of crusaders opposed to the detour to Zara, informed the Zarans that the crusade leaders had a letter from Pope Innocent III threatening to excommunicate anyone who raised a sword against Zara. Simon insisted that if the citizens could defend themselves against the Venetians they would be safe from the Frankish (non-Venetian) crusaders, who would not disobey the pope. The delegates thanked Simon and returned to their city. When Dandolo and the crusader barons returned they were outraged by these actions. A peaceful surrender had been thwarted.
The pope's stern letter forced the crusade's leaders to choose between excommunication, for attacking a city under church protection, and the end of the crusade. Believing that God could not desire the latter, most chose to keep their word to the Venetians as a matter of honor. Simon and his men withdrew from the army, but the majority of the crusaders attacked Zara, capturing it on 24th November-as a result, the Fourth Crusade was excommunicated.
The Frankish leaders sent a delegation to Innocent III, begging forgiveness. He granted them the absolution they sought, but reaffirmed the excommunication of the Venetians. The pope was now convinced that the Venetians had deliberately taken over the crusade for their own ends. In a letter to the crusade leaders he said that once the Franks had been delivered to the Holy Land, they should have nothing more to do with the Venetians.
The crusade had other problems too, with huge debts, no money, and a shortage of provisions. According to the contract, Venice supplied each man with enough to sustain him at low activity levels for about nine months. Since they had begun eating their provisions in late June 1202 the crusaders would have been out of food by late March 1203, when the fleet was again ready to sail from Zara. There were insufficient resources to keep the army and fleet together, let alone support it on its mission to fight in Egypt.
It was at this moment that a group of envoys arrived at Zara led by a Byzantine prince, Alexius Angelus, who had recently fled to the West. His father, the emperor Isaac II Angelus, had been blinded and deposed by his own brother, Alexius III, in 1195. The young man asserted that he, not his usurping uncle, was the rightful emperor of Constantinople. If the crusaders would help him to his throne he would provide them with food, pay them 200,000 silver marks, join their crusade with 10,000 soldiers, place a permanent garrison in the Holy Land, and restore the obedience of the Greek church to Rome. For the crusaders this offer was extremely attractive. But it would, of course, necessitate a further diversion of the troubled crusade.
There was considerable debate among the crusaders concerning the offer made by Alexius Angelus. The majority of the troops wanted no more detours or delays. They had made vows to fight for Christ, not a Byzantine pretender. However, the crusade leaders favored helping the young man. They saw that with only a few months left on the fleet's lease, no food, and crushing debt, the crusade simply could not survive without replenishing its resources. It would have made little sense to transport an impoverished army directly to the East. They also saw the detour to Constantinople as an errand of mercy to free the Byzantine people from the oppression of a tyrant. Alexius Angelus assured them that his uncle, the emperor, was so hated in the city that he would be overthrown as soon as the crusaders arrived with the rightful heir.
The crusade leaders accepted, informing the pope shortly afterward: "lacking all foodstuffs and supplies, we appeared to be bearing a burden to the Holy Land... rather than bringing some sort of aid; nor did we believe that, given such extreme poverty, we could effectively land in the territory of the Saracens. “When the rank-and-file soldiers learned of the leaders' action, many of them abandoned the crusade, making their own way to the East to fulfill their vows. Only by swearing that the stop in Constantinople would be brief were the leaders able to win the grudging acceptance of the other crusaders.
The crusade left Zara in April 1203, made its way through the Aegean and arrived at Constantinople in late June. Mismanagement had reduced Byzantium's once proud navy to a few worm-eaten vessels incapable of challenging the enormous crusade fleet. In several dramatic displays, the crusaders let the people of Constantinople know that they came as friends, having brought them their rightful lord. The Byzantines responded with insults, rocks, and bare backsides. They wanted nothing to do with the Westerners' pretender.
Reluctantly, the crusaders at last accepted that they would have to attack. The massive city had enormous fortifications that no enemy had ever breached before and a garrison three times the size of the crusader force. Nevertheless, on 17th July the crusaders attacked the northeastern area of the city, the Franks assaulting the land wall and the Venetians the seawall. After fierce fighting the Venetians captured a portion of the wall and entered a short distance before being pushed back by the elite imperial bodyguard.
Discontent at Alexius III's ineffectiveness made him fearful of a coup and he fled. His brother, Isaac II Angelus, was freed and restored to the throne. He ordered the gates to be opened so that Prince Alexius could enter. The crusaders were dutifully acclaimed as heroes and within days the young man was crowned co-emperor Alexius IV