Theodore II Laskaris, emperor of Nicea (Byzantine emperor in exile)
Theodore II Doukas Laskaris or Ducas Lascaris (Greek: Θεόδωρος Δούκας Λάσκαρις, romanized: Theodōros Doukas Laskaris; 1221/1222 – 16 August 1258) was Emperor of Nicaea from 1254 to 1258. He was the only child of Emperor John III Doukas Vatatzes and Empress Irene Laskarina. His mother was the eldest daughter of Theodore I Laskaris who had established the Empire of Nicaea as a successor state to the Byzantine Empire in Asia Minor, after the crusaders captured the Byzantine capital, Constantinople, during the Fourth Crusade in 1204. Theodore received an excellent education from two renowned scholars, Nikephoros Blemmydes and George Akropolites. He made friends with young intellectuals, especially with a page of low birth, George Mouzalon. Theodore began to write treatises on theological, historical and philosophical themes in his youth.

Emperor John III arranged for Theodore to marry Elena of Bulgaria in 1235, to forge an alliance with her father, Ivan Asen II, against the Latin Empire of Constantinople. According to Theodore himself, their marriage was happy, and they had five or six children. From 1241, Theodore acted as his father's lieutenant in Asia Minor during his frequent military campaigns in the Balkan Peninsula. From around 1242, he was his father's co-ruler, but was not crowned as co-emperor. During this period, his relationship with some prominent aristocrats, particularly Theodore Philes and Michael Palaiologos, grew tense.

Theodore succeeded his father on 4 November 1254. He dismissed many high officials and army commanders of aristocratic origin, replacing them with loyal friends, including those of low birth. He made a defensive alliance with Kaykaus II, the Seljuk Sultan of Rum, against the Mongol Empire. He repelled a Bulgarian invasion of Thrace and Macedonia and forced Michael II Komnenos Doukas, the ruler of Epirus, to cede Dyrrachium on the coast of the Adriatic Sea to Nicaea. He reformed the military, recruiting more soldiers from among the native peasantry of Asia Minor. Eventually, Michael II of Epirus forged an alliance with Stefan Uroš I, King of Serbia, and Manfred of Sicily against Nicaea. Theodore's newly appointed generals could not resist their joint invasion in 1257. Theodore fell seriously ill and could rarely take part in state administration during the last months of his life. He appointed George Mouzalon regent for his underage son, John IV, before he died of either chronic epilepsy or cancer. In ten days, Mouzalon fell victim to an aristocratic plot, and Michael Palaiologos assumed the regency, usurping the throne soon after.