(1438–45) Ecumenical council held in an attempt to reunify the Eastern Orthodox and Roman Catholic churches.
It was called by Pope Eugenius IV; the Eastern church was represented by Emperor John VIII Palaeologus and others. Fear of facing the Turks without Western support led the Eastern participants to sign the Decree of Union (1439), but on their return to Constantinople most renounced it. Union was officially declared in Hagia Sophia in 1452, but the following year the Ottoman Empire captured Constantinople, and the few partisans of the union fled. In 1448 a council of Eastern bishops condemned it officially. General support for the council and its goals strengthened the authority of the papacy and contributed to the failure of the conciliar movement .