The Papacy in the Late Middle Ages
Council of Pisa
In 1409, cardinals on both sides managed to arrange a General Council at Pisa. It was an imposing affair, with over 500 prelates in attendance. Both popes were deposed as schismatics and heretics, and the Council elected a new pope, Alexander V (1409-1410).
Gregory XII and Benedict XIII had opposed the calling of this council, though, and they promptly denounced and excommunicated Alexander. The Council refused to back down, and now there were three popes.
Alexander was succeeded by John XXIII (1410-1415), and the situation remained unchanged. The failure of the Council of Pisa led immediately to calls for another Council, but the question of authority was now worse than ever.
From all quarters came the call for another council. The spectacle of three popes, three Colleges of Cardinals, three bodies of curia, was too much to bear. The only figure within Christendom with sufficient prestige, if not authority, was that of the Emperor.
Emperor Sigismund was not an especially strong or dynamic ruler, but he was as concerned as any Christian with the miserable state of affairs. With the failure at Pisa, he at last responded to entreaties and called another general council at the Imperial city of Constance. It is a nice irony that the rivalry among popes was solved only upon the instigation and authority of their ancient rival, the Holy Roman Emperor.