In 1204, Byzantine emperor Alexius V Ducas Murtzouphlos fled Constantinople after crusaders invaded the city. Theodore I Lascaris, the son-in-law of Emperor Alexius III Angelus, was proclaimed emperor, but he too fled, to the city of Nicaea in Bithynia, realizing the situation in Constantinople was hopeless.
The Latin Empire, which was established by the Crusaders in Constantinople, had poor control over former Byzantine territory, and Byzantine successor states sprang up in Epirus and Trebizond as well as Nicaea, although Trebizond broke away as an independent state a few weeks before the fall of Constantinople. Nicaea, however, was the closest to the Latin Empire and was in the best position to attempt to re-establish the Byzantine Empire. Theodore Lascaris was not immediately successful, as he was defeated at Poemanenum and Prusa in 1204, but he was able to capture much of northwestern Anatolia after the Latin Emperor Baldwin I had to defend against invasions from Kaloyan of Bulgaria. Theodore also defeated an army from Trebizond, as well as other minor rivals, leaving him in charge of the most powerful of the successor states. In 1206, Theodore proclaimed himself emperor at Nicaea.