The practice of bringing a pretender to secure local support for an invasion was well-established among the western powers. But on this occasion the pretender was exactly who he claimed to be. Moreover, for a while at least, the Venetians seemed satisfied that Alexius IV would deliver what he had promised. That is, they were confident that he would pay them a substantial sum, equal to that agreed in April 1201 as payment for transportation of the crusaders to Egypt. However, within a year of his installation, in February 1204, Alexius IV was murdered by a faction opposed to his succession, provoking an assault on the walls of Constantinople in April 1204. Despite their failure effectively to breach the city’s fortifications, the crusaders were able to enter the city in full force. The reason that it was thrown open to the crusaders was the inability of those in power in the city to stand together and launch a solid counter-offensive. And the reason for that was factionalism within the ruling elite in Constantinople.

From Paul Stephenson’s Byzantium’s Balkan Frontier: A Political Study of the Northern Balkans, 900-1204, Cambridge: Cambridge University Press, 2000, p. 279.