Following the fall of Constantinople in 1204 much of the Byzantine Empire was partitioned amongst newly created Crusader states. Latin rule in the east was never secure, and some of that instability derived from the concessions made to the increasingly dominant Venetians in order to secure their assistance in the Fourth Crusade. Two surviving outposts of Byzantine rule, founded by Byzantine aristocrats who had fled Constantinople after the Fourth Crusade, remained: Nicaea and Epirus. In 1224 the despotate of a Epirus conquered the Latin kingdom of Thessalonika. The Empire of Nicaea began expanding its territory in 1225, under the leadership of John III Doukas Vatatzes. John III formed an alliance with the Second Bulgarian empire over the next 20 years and gradually reduced the power of the Latin Empire, which held onto Constantinople until 1261. To the west of the Latin Empire the short-lived kingdom of Thessalonika was annexed by the Empire of Nicaea in 1246 after an internal power struggle. Nicaea became increasingly powerful and assertive. The Seljuk Empire on Nicaea’s eastern frontier was coming under attack from the Mongols, which allowed Nicaea to focus pressure on its western rivals and eventually reunite the former heartland of the Byzantine Empire in 1261 when the Emperor of Nicaea, Michael Palaeologus, recaptured Constantinople, re-establishing the Byzantine Empire.
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