In 1204, Byzantine emperor Alexios V Ducas Murtzouphlos fled Constantinople after crusaders invaded the city. Soon after, Theodore I Lascaris, the son-in-law of Emperor Alexios III Angelos, was proclaimed emperor but he too, realizing the situation in Constantinople was hopeless, fled to the city of Nicaea in Bithynia.

Theodore Lascaris was not immediately successful, as Henry of Flanders defeated him at Poimanenon and Prusa (now Bursa) in 1204. But Theodore was able to capture much of northwestern Anatolia after the Bulgarian defeat of Latin Emperor Baldwin I in the Battle of Adrianople, because Henry was recalled to Europe to defend against invasions from Tsar 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 1205, he assumed the traditional titles of the Byzantine emperors. Three years later, he convoked a Church council to elect a new Orthodox patriarch of Constantinople. The new patriarch crowned Theodore emperor and established his seat at Theodore's capital, Nicaea.