In 1455 Carvajal was sent by Pope Callistus III to Hungary to preach a vigorous crusade against the Turks, and for six years was the leader of the first effectual resistance made by Christian Europe to the progress of the Ottoman conquerors of Constantinople (1453). Aided by the Observant Franciscan preacher John Capistran, he gathered an army of about 40,000 men, effected a union with the troops of John Hunyadi, and on 22 July 1456 the siege of Belgrade, the key of the Danube, was raised by a victory that inaugurated the century-long resistance of Hungary to the Ottomans. In 1457, Carvajal baptized King Thomas of Bosnia into the Roman Catholic Church.