Constantinople fell to Mehmed II in May 1453, and the shock ran through Latin Christendom for a generation. Alfonso de Borja was elected pope in April 1455 with the recovery of the city as the declared purpose of his pontificate, and he pursued it with a single-mindedness his contemporaries found either admirable or obsessive. He sold papal property and plate, cut the expenditure of the curia, had a fleet built at Ostia, and sent legates across Europe preaching a crusade to rulers who had, almost without exception, more pressing quarrels at home.
The one substantial result came in the summer of 1456 and not at Constantinople. Ottoman forces moved up the Danube to besiege Belgrade, the gateway to Hungary, and the defence was organized by the Hungarian commander John Hunyadi with a large force of poorly armed volunteers raised by the Franciscan preacher John of Capistrano, who had been sent out by the pope. The siege was broken in July. Callixtus had already ordered church bells rung at noon across Christendom as a call to pray for the defenders, and the noon bell is often said to commemorate the victory afterwards, though the order preceded the news.
He did not live to see anything further. The crusade dissolved after his death in 1458, and the Ottoman advance into the Balkans continued. What the pontificate demonstrated was the limit of papal power to move fifteenth century Europe: the money, the preaching and the legates were real, and they produced one successful defensive battle rather than the reconquest he had promised.