Byzantine emperor (1425-1448). John’s long and eventful reign over a truncated empire, consisting of Constantinople (mod. Istanbul, Turkey) plus environs and the despotate of the Morea, witnessed the last major attempt at church union and the last massive crusade of the West against the Ottoman Empire.
Co-emperor with his father Manuel II from 1421, John experienced the military advance of the Ottomans in Greece: the siege of Constantinople (1422), invasions of the Morea (1423 and 1431), and the capture of Thessalonica and Ioan- nina (1430); during his visit to Venice and Hungary (1423), he desperately tried to secure Western help, a policy that he pursued more intensely following his father’s death (1425). In 1426-1427 he led a successful campaign in the Morea against Carlo I Tocco, lord of Kephallenia and Epiros, who had seized the fortress of Glarentza, while in 1429-1430 he took part in the campaign of his brother Constantine, despot of Morea, which captured Patras, thus ending the rule of the Latin principality of Achaia in the Peloponnese.
Following negotiations in 1437 with Pope Eugenius IV, John led the Byzantine delegation to the Council of Ferrara- Florence (1438-1439), yet, despite the official proclamation of union of the Latin and Greek churches (July 1439), his policy was never implemented in Byzantium due to staunch opposition led by the anti-unionists. Moreover, the fleeting hopes of a decisive Western crusade against Ottoman expansion in the Balkans, launched in 1443 under King Ladislas I of Hungary, were checked following the defeat of the Crusade of Varna (November 1444) at the hands of Sultan Murad II.
The sultan’s aged general Turakhan Beg then conducted a punitive expedition in southern Greece and the Morea (1446), forcing the despot Constantine to become his vassal. Finally, news of the defeat of John Hunyadi by the Ottomans at the second battle of Kosovo (1448) struck the final blow against the frustrated John, who died on 31 October of the same year.