Theodore Laskaris, Empire of Nicaea, the center of the Orthodox church and imperial government in exile, had in 1214 concluded a peace treaty with the Latin Empire at Nymphaion, and in 1219 he had married a niece of Emperor Henry.

In spite of predominantly peaceful relations, Theodore had attacked the Latin Empire again in 1220, but peace was restored after he negotiates a settlement with Robert of Courtenay, fourth emperor of the Latin Empire of Constantinople, to whom he betroths his daughter Eudocia.

By his courage and military skill, he has enabled the imperial Greek state not merely to survive, but ultimately to beat back the Latin invasion.

The ruler of a territory roughly coterminous with the old Roman provinces of Asia and Bithynia, Theodore dies in November 1221.

John Doukas Vatatzes, a successful soldier from a military family, is 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.

The Vatatzes family had first become socially prominent in the Komnenian period and had forged early imperial connections when Theodore Vatatzes married the porphyrogenete princess Eudokia Komnene, daughter of Emperor John II Komnenos.

John had been chosen by Theodore as husband for his daughter Irene and as heir to the throne, excluding members of the Laskarid family from the succession.

He ascends the throne of Nicaea as Emperor John III in mid-December 1221.