Emperor of Nicaea (1222-1254), and major opponent of the Latin empire of Constantinople.
John was born around 1192 and succeeded to the throne of Nicaea on the death of Emperor Theodore I Laskaris, whose daughter Irene he had married. John followed a successful economic policy based on autarky, which led to prosperity for both the state and the people of the empire. That prosperity enabled him to assemble a powerful fleet and buy the services of Western mercenaries. In a series of successful military campaigns against the Latin empire of Constantinople and the principality of Epiros, he expanded the territorial possessions of the Nicaean Empire into Thrace and Macedonia. In 1234-1236 he besieged Constantinople with the help of his ally, the Bulgarian tsar Ivan Asen II, but without success. In 1232 John encouraged Ger- manos II, the exiled Greek patriarch of Constantinople, to open negotiations with the papacy on the issue of the reunification of the Greek Orthodox and Latin churches. John believed that the pope was prepared to withdraw his support from the Latin Empire if the two churches agreed on reunification. For that reason he took an active part in the negotiations, which lasted to the end of his reign, but in the end no agreement was signed. At the same time that he was negotiating with Rome, John was working on strengthening his military and financial relationship with Frederick II, Holy Roman Emperor and king of Sicily, whose daughter Constance (Greek name: Anna) he married (1240/1241).
John died at Nymphaion (mod. Kemalpaşa, Turkey) in Asia Minor on 4 November 1254, leaving the Empire of Nicaea as the strongest power in the region. In the seventeenth century he was canonized by the Greek Orthodox Church.