It is not necessary to detail the causes which led to the diversion of the fourth Crusade upon Constantinople. Venice, it is enough to say, betrayed the Christian cause by a secret treaty with the infidel, and then formed a plot for the capture of the city. Alexius III. had deposed Isaac Angelus in 1195; his son Alexius was allowed to escape and secretly took ship for Italy and eventually threw himself upon the charity of his brother-in-law, Philip of Swabia, the claimant of the imperial crown of the West. He was assisted; and by a series of complicated intrigues the Crusaders were induced to undertake the capture of Constantinople and the restoration of the Empire to the supposed rightful heir, as a step towards the accomplishment of the duty to which they were pledged, the recovery of the Holy Land. The Pope's wishes were set aside, the honest leaders were hoodwinked, and Dandolo won the day.
On the 23rd of June 1204 the crusading fleet anchored at San Stefano. Thence they saw the magnificent city that lay before them. "Be sure," says Villehardouin, "there was not a man who did not tremble, because never was so great an enterprise undertaken by so small a number of men." Next day they sailed up to the Bosphorus, past the walls, crowded with spectators, to the anchorage of Chalcedon. The Emperor sent to know their intentions: they ordered him to surrender the crown to the young Alexius. Then came another of those picturesque scenes of which the mediæval history of the New Rome is so full. It was determined to show the young prince to the people whom he came to recover to their allegiance. The splendid Venetian galleys sailed up to the walls of the Sea of Marmora, and stopped where the crowds that thronged them could see. Then loud voices proclaimed the presence of the young Alexius, and demanded the loyal assent of the people to the restoration of his father. Only mocking laughter came back from the walls.
Then the Crusaders prepared for the attack. First it was necessary to break the chain which crossed the Golden Horn from Galata, near what is now Tophané, to near the point of the peninsula of Byzantium. A fierce attack was made on the watch-tower at Galata, from which the chain began. It was captured, the chain was loosed, and the fleet sailed up the Golden Horn. The army was then landed beyond the walls, where is now Eyoub, and took up a position opposite the Blachernae quarter, which had so long been felt to be the weakest point. They were opposed then by the wall of Manuel Comnenus which extended southward of the wall of Heraclius, and considerably in advance of the old Theodosian fortification. Moats, walls, towers, stood before them, a defence hitherto unbroken, and which even before the last fortification was erected it had been found impossible to overthrow.