John III Doukas Vatatzes, latinized as Ducas Vatatzes (Greek: Ιωάννης Γ΄ Δούκας Βατάτζης, Iōannēs III Doukas Vatatzēs, c. 1192, Didymoteicho – 3 November 1254, Nymphaion) was emperor of Nicaea 1221–1254.
John Doukas Vatatzes was probably the son of the general Basileios Vatatzes, Duke of Thrace, who died in 1193, and his wife, an unnamed daughter of Isaakios Angelos and cousin of the Emperors Isaac II Angelos and Alexios III Angelos. He had two older brothers. The eldest was Isaakios Doukas Vatatzes (died 1261), who married and had two children: Ioannes Vatatzes (born 1215), who married to Eudokia Angelina and had two daughters Theodora Doukaina Vatatzaina, wife of Michael VIII Palaiologos, and Maria Vatatzaina, married to Michael Doukas Glabas Tarchaneiotes, Military Goveror of Thrace; and a daughter, married to Konstantinos Strategopoulos. His other older brother was the father in law of Alexios Raul (died 1258).
A successful soldier from a military family, in 1212, John was chosen by Emperor Theodore I Laskaris as husband for his daughter Irene Laskarina and as heir to the throne. This arrangement excluded members of the Laskarid family from the succession, and when John III Doukas Vatatzes became emperor in mid-December 1221, he had to suppress opposition to his rule. The struggle ended with a battle in 1224, in which his opponents were defeated in spite of the support they had acquired from the Latin Empire of Constantinople. John III's victory led to territorial concessions by the Latin Empire in 1225, but was followed by John's incursion into Europe, where he seized Adrianople.
John III's possession of Adrianople was terminated by Theodore Komnenos Doukas of Epirus and Thessalonica, who drove the Nicaean garrison out of Adrianople and annexed much of Thrace in 1227. The elimination of Theodore by Ivan Asen II of Bulgaria in 1230 put an end to the danger posed by Thessalonica, and John III made an alliance with Bulgaria against the Latin Empire. In 1235 this alliance resulted in the restoration of the Bulgarian patriarchate and the marriage between Elena of Bulgaria and Theodore II, respectively Ivan Asen II's daughter and John III's son. In the same year the Bulgarians and Nicaeans campaigned against the Latin Empire, and in 1236, they attempted a siege of Constantinople. Subsequently, Ivan Asen II adopted an ambivalent policy, effectively becoming neutral, and leaving John III to his own devices.
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