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.

Source: Wikipedia — Siege of Constantinople (1235–1236) (CC BY-SA 4.0)