Emperor John III Doukas Vatatzes of Nicaea concluded an alliance with Bulgaria, which in 1235 resulted in a joint campaign against the Latin Empire.

In 1235, Angelo Sanudo, the second Duke of the Archipelago, sent a naval squadron for the defense of Constantinople, where the Emperor John of Brienne was being besieged by John III Doukas Vatatzes, Emperor of Nicaea, and Ivan Asen II of Bulgaria.

Ivan Asen II and Vatatzes agreed to continue the siege in the next year, but the Bulgarian Emperor later refused to send troops.

With the death of John of Brienne in 1237, the Bulgarians broke the treaty with Vatatzes because of the prospect that Ivan Asen II could become a regent of the Latin Empire [citation needed].

By 1247, the Nicaeans had effectively surrounded Constantinople, with only the city's strong walls holding them at bay, and the Battle of Pelagonia in 1258 signaled the beginning of the end of Latin predominance in Greece.