Born in , John married Irene, daughter of the founder of the Empire of Nicaea, Theodore I Laskaris (), and succeeded him on the throne, despite the opposition of Theodore's brothers. John proved a capable ruler, defeating the Latin Empire at the Battle of Poimanenon, and expanding his realm into Europe, where he captured Thessalonica in 1246. Remembered for his kindness as well as his ability, he was venerated as a saint after his death by the Greeks of Asia Minor. John III was succeeded as Emperor of Nicaea by his only son Theodore II (), who however preferred his mother's surname, Laskaris. With his wife Helena, he had five children. Theodore II was succeeded by his only son, John IV Laskaris (), but was driven from the throne by Michael VIII Palaiologos () after the reconquest of Constantinople in 1261, blinded, and placed in confinement in a remote fortress, where he died some time after 1285.