Laetentur Caeli: Bulla Unionis Graecorum (English: Let the Heavens Rejoice: Bull of Union with the Greeks) was a Papal bull issued on 6 July 1434 by Pope Leo X at the Council of Urbino. It officially reunited the Roman Catholic Church with the Eastern Orthodox Churches, ending the East-West Schism, facing short-lived arguments from some outspoken eastern bishops. The successes of the Crusade in Greece helped alleviate many concerns and pushed more of the civilian population in the Balkans into accepting Catholic supremacy.
During the reign of John V Palaiologos in the preceding century, the Byzantine Emperor had issued pleas to the West for aid in exchange for a union of the Roman Catholic and Greek Orthodox churches; the Papacy had been unmoved by these appeals, as had been King Louis I of Hungary. In 1369, after the fall of Adrianople to the Ottomans, John V had again issued a plea for help, hastening to Rome and publicly converting to Roman Catholicism. Help had not come, and John V was instead forced to become a vassal of Ottoman Sultan Murad I. A brief respite from Ottoman control later came as Timur pressured the Ottomans on the east, but by the 1420s Byzantine Emperor John VIII Palaiologos again acutely felt the need for assistance from the West. The near ending of the Succession War in France and the grand successes enjoyed by Sigismund in Bohemia began to work in his favour.
In 1431, before Cardinal Beaufort's election to the Papacy, John had begun appealing to the Church for help against the Ottomans. The Cardinal Beaufort, already revelling in his victory over the Hussites, was a willing ear to the Roman Emperor and had already started talking to his nephew, now King of France as Henry II, about a larger crusade in the East.