The History of the Decline and Fall of the Roman Empire, by Edward Gibbon

Chapter LX: The Fourth Crusade

Schism Of The Greeks And Latins.—State Of Constantinople.—Revolt Of The Bulgarians.—Isaac Angelus Dethroned By His Brother Alexius.—Origin Of The Fourth Crusade.—Alliance Of The French And Venetians With The Son Of Isaac.—Their Naval Expedition To Constantinople.—The Two Sieges And Final Conquest Of The City By The Latins.