John VIII Palaiologos or Palaeologus (; 18 December 1392 – 31 October 1448) was the penultimate Byzantine emperor, ruling from 1425 to 1448. Biography John VIII was the eldest son of Manuel II Palaiologos and Helena Dragaš, the daughter of the Serbian prince Constantine Dragaš. He was associated as co-emperor with his father before 1416 and became sole emperor on 1 July 1425, although he had already assumed full power on 19 January 1421. In June 1422, John VIII Palaiologos supervised the defense of Constantinople during a siege by Murad II, but had to accept the loss of Thessalonica, which his brother Andronikos had given to Venice in 1423. To secure protection against the Ottomans, he made two journeys to Italy in 1423 and 1439. In 1423, he became the last Byzantine emperor (the first since emperor Constans II' visit in 663) to make a visit to Rome. During the second journey he visited Pope Eugene IV in Ferrara and consented to the union of the Greek and Roman churches. The union was ratified at the Council of Florence in 1439, which John attended with 700 followers including Patriarch Joseph II of Constantinople and George Gemistos Plethon, a Neoplatonist philosopher influential among the academics of Italy. The union failed due to opposition in Constantinople, but through his prudent conduct towards the Ottoman Empire he succeeded in holding possession of the city. John VIII Palaiologos named his brother Constantine XI, who had served as regent in Constantinople in 1437–1439, as his successor. Despite the machinations of his younger brother Demetrios Palaiologos his mother Helena was able to secure Constantine XI's succession in 1448. John VIII died at Constantinople in 1448, becoming the last reigning Byzantine emperor to die of natural causes, and was buried in the Pantokrator Monastery. Marriages John VIII Palaiologos was married three times. His first marriage was in 1414 to Anna of Moscow, daughter of Grand Prince Basil I of Moscow (1389–1425) and Sophia of Lithuania. She died in August 1417 of plague. The second marriage, arranged by his father Manuel II and Pope Martin V, was to Sophia of Montferrat in 1421. She was a daughter of Theodore II, Marquess of Montferrat, and his second wife Joanna of Bar. Joanna was a daughter of Robert I, Duke of Bar, and Marie de Valois. Her maternal grandparents were John II of France and Bonne of Bohemia. His third