Siege of Constantinople (1235–1236)

The siege of Constantinople took place between 1235 and late 1236 when a joint Bulgarian–Nicaean army led by Tsar Ivan Asen II of Bulgaria and Emperor of Nicaea John III Doukas Vatatzes conducted an unsuccessful siege of the capital of the Latin Empire.

The siege arose from the prolonged conflict between the Latin Empire, Bulgaria and the Byzantine successor states, the Empire of Nicaea and Despotate of Epirus, following the capture and sack of Constantinople by the Fourth Crusade in 1204. Each aimed to revive or establish an empire in the territories of the Byzantine Empire. Initially, Asen was led by the Latins to believe he would become emperor of Constantinople following the death of Robert of Courtenay in 1228. The Latins terminated negotiations after the Bulgarians defeated Epirus at the Battle of Klokotnitsa in 1230, as the Latins sought only to prevent an alliance between Bulgaria and Epirus. The announcement that John of Brienne had become the new Latin emperor angered Asen. He initiated negotiations and eventually agreed to a formal alliance with Vatatzes in early 1235 to combine their forces to reclaim Constantinople. Ecclesiastical concessions and a dynastic marriage sealed the alliance.

Despite their combined strength against a small garrison, the siege failed to breach the Constantinopolitan walls supported by the Venetian navy. The allied army withdrew in the autumn for the winter and renewed the siege in 1236. Later that year, Asen shifted allegiance, presumably to avoid calls for a crusade from the pope or to capture the city, before abandoning the Latin Empire a year later and refraining from more campaigns. While the siege did not topple the Latin Empire, it increased pressure against Latin rule and foreshadowed the eventual Byzantine recovery of Constantinople in 1261 by the Nicaean forces.