Theodore was born to Emperor John III Doukas Vatatzes and Irene Laskarina in the imperial palace in Nicaea in late 1221 or early 1222. Irene was the eldest daughter of Theodore I Laskaris, the founder of the Empire of Nicaea. Nicaea was a successor state of the Byzantine Empire, established after the West European ("Latin") crusaders captured Constantinople during the Fourth Crusade in 1204. Vatatzes was Irene's third husband. He was born into a Byzantine aristocratic family related to the imperial dynasties of Komnenos and Angelos. When he wed Irene, he served as the commander of the imperial guard. Although Irene's two brothers had died prematurely, Theodore I did not name Vatatzes as his official heir. He most likely expected that his third wife, the Latin Maria of Courtenay, would bear sons, but he did not father children with her. Theodore I died in November 1221. Vatatzes had to defeat the late emperor's brothers, Alexios and Isaac Laskaris, to seize the throne. His victory over his wife's uncles and their Latin allies in the Battle of Poimanenon in the winter of 1223–1224 opened the way for the reconquest of Byzantine territories lost to the Latins in Asia Minor.