Florentine Union

The Seventeenth Ecumenical Council of the Roman Catholic Church was convoked as the Council of Basel (Basle in the once-preferred English spelling) by Pope Martin V shortly before his death in February 1431 and took place in the context of the Hussite wars in Bohemia and the rise of the Ottoman Empire. At stake was the greater conflict between the Conciliar movement and the principle of papal supremacy.

The Council entered a second phase after Emperor Sigismund's death in 1437. Pope Eugene IV convoked a rival Council of Ferrara on 8 January 1438 and succeeded in drawing the Byzantine ambassadors to Italy. The Council of Basel first suspended him, declared him a heretic, and then in November 1439 elected an antipope, Felix V. The rival Council of Florence (moved to avoid plague in Ferrara) concluded in 1445 after negotiating unions with the various eastern churches. This bridging of the Great Schism proved fleeting, but was a political coup for the papacy. In 1447, Sigismund's successor Frederick III commanded the city of Basel to expel the Council of Basel; the rump council reconvened in Lausanne before dissolving itself in 1449.

The initial location at Basel reflected the desire among parties seeking reform to meet outside the territories of the Papacy, the Holy Roman Empire or the kings of Aragon and France, whose influences the council hoped to avoid. Ambrogio Traversari attended the Council of Basel as legate of Pope Eugene IV.