The rulers of Thessalonica bore the imperial title from 1225/7 until 1242, when they were forced to renounce it and recognize the suzerainty of the rival Empire of Nicaea.

The Latin Empire was set up in Constantinople itself, while most of northern and eastern mainland Greece went to the Kingdom of Thessalonica under Boniface of Montferrat.

From the start, Manuel's suzerainty was rather theoretical, and by 1236–37 Michael was acting as an independent ruler, seizing Corfu, and issuing charters and concluding treaties in his own name.

The latter had been released from captivity and secretly returned to Thessalonica after John II Asen fell in love with and married his daughter Irene.

After he captured Larissa, Theodore offered him a settlement, whereby he and his son would keep Thessalonica, Manuel would keep Thessaly, while another brother, Constantine Komnenos Doukas, would rule over Aetolia and Acarnania, which he had held as an appanage since the 1220s.

[19] In 1241, on the assurance of safe conduct, Theodore went to Nicaea, but there Vatatzes held him prisoner, and in the next year he embarked with his army for Europe and marched on Thessalonica.

[22] After this remarkable success, Vatatzes turned on Thessalonica, where leading citizens were already conspiring to overthrow Demetrios and deliver the city to him.

Thessalonica was incorporated into the Nicaean state, with Andronikos Palaiologos as its governor, while Demetrios was sent to a comfortable exile in estates granted to him in Asia Minor.

Michael tried to capture Thessalonica and re-establish a strong western Greek state able to challenge Nicaea for supremacy and the Byzantine imperial inheritance.

This did not long deter Michael, who after 1257 sought alliances with other powers against the growing menace of Nicaea, including the Latin Principality of Achaea and Manfred of Sicily.

More importantly, the victory opened the way for the Nicaean recapture of Constantinople on 15 August 1261, and the restoration of the Byzantine Empire under the Palaiologos dynasty.