John III (John Ducas Vatatzes)do͝o´kəs vətăt´zēz [key], d. 1254, Byzantine emperor of Nicaea (1222–54), successor and son-in-law of Theodore I. He extended his territory in Asia Minor and the Aegean islands but failed (1235) to take Constantinople from the Latins, although he was aided by Ivan II of Bulgaria. Subsequently Ivan, the Cumans, and the Latins of Constantinople allied themselves against John, who held his own. John joined the Turks against the Mongol invaders. He annexed Salonica (Thessaloníkai) in 1246 and reduced the despotat of Epirus to vassalage, thus nearly recovering the territories of the Byzantine Empire. He maintained close relations with the German emperor, Frederick II, whose daughter he married. During his reign the empire flourished. He was succeeded by his son Theodore II.
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