The empire of Nicaea began expanding its territory in 1225 under the leadership of John III Doukas Vatatzes. The first territories he acquired were ceded from the rival Latin Empire, immediately to the north, following an unsuccessful coup against him, for which the Latin Empire had provided support. 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 on to Constantinople until 1261. To the west of the Latin Empire was the short-lived empire of Thessalonika which, after an internal power struggle, was annexed by the empire of Nicaea in 1246 along with other territories in the region belonging to the Second Bulgarian Empire. The Seljuk Empire on Nicaea’s eastern frontier was coming under attack from the Mongol Empire which allowed Nicaea to focus pressure on its western rivals and eventually reunite the former heartland of the Byzantine Empire in 1261.
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