[6] As a result, when John III became emperor in December 1221,[a] following Theodore I's death in November,[9][10] he had to suppress opposition to his rule by Laskaris' brothers, Alexios and Isaac.

[13] Rubruck was critical of the Hellenic traditions he encountered in the Empire of Nicaea, specifically the feast day for Saint Felicity favored by John Vatatzes, which Friedrich Risch suggests would have been the Felicitanalia, practiced by Sulla to venerate Felicitas in the 1st century with an emphasis on inverting social norms, extolling truth and beauty, reciting profane and satirical verse and wearing ornamented "cenatoria", or dinner robes during the day.

[14] In spite of some reverses against the Latin Empire in 1240, John III was able to take advantage of Ivan Asen II's death in 1241 to impose his own suzerainty over Thessalonica (in 1242), and later to annex this city, as well as much of Bulgarian Thrace in 1246.

[19] In the face of Bulgarian neutrality, John III sought allies elsewhere, turning to the Holy Roman Emperor Frederick II von Hohenstaufen.

At the Council of Lyon in 1245, Frederick was deposed as emperor and excommunicated, for, among a multitude of other reasons, marrying his daughter to John III who was called by Innocent IV "that enemy of God and the church.

[27] John III continued to send troops, including archers and infantryman, and subsidies to Italy via Epirus between 1247 and 1250 to aid his father-in-law, who finally triumphed over the papacy in the Battle of Cingoli in August 1250, however Frederick died of dysentery in December the same year.

However, Constance-Anna refused and moved to her brother's court in Sicily in 1261, thus severing all ties with Byzantium and formally ending the alliance started by John III and Frederick II.

[34][35][36][37][38] John III effected Nicaean expansion into Europe, where by the end of his reign he had annexed his former rival Thessalonica and had expanded at the expense of Bulgaria and Epirus.

He also expanded Nicaean control over much of the Aegean and annexed the important island of Rhodes,[39] while he supported initiatives to free Crete from Venetian occupation aiming toward its re-unification with the Byzantine empire of Nicaea.

[40] Styling himself the true inheritor of the Roman Empire, John III encouraged justice and charity, and provided active leadership in both peace and war despite his epilepsy.

[45] Alice Gardiner remarked on the persistence of John's cult among the Ionian Greeks as late as the early 20th century, and on the contrast she witnessed where "the clergy and people of Magnesia and the neighbourhood revere his memory every fourth of November.

"[46] His feast day is formally an Eastern Orthodox holiday, although it is not commemorated with any special liturgy; there are two known historical akolouthiai for him, including an 1874 copy of an older Magnesian menaion for the month of November, which shows that in the 15th and 16th century, he was venerated as "the holy glorious equal of the Apostles and emperor John Vatatzes, the new almsgiver in Magnesia.

"[47] The relevant hymns are preserved in only one known manuscript in the library of the Leimonos monastery on Lesbos, Greece, and include references to the feast day for the almsgiver John Vatatzes.