Crusade of St. John of Capistrano. The Crusade of St. John of Capistrano (1456), also known as the Siege of Belgrade of 1456, began after the fall of Constantinople in 1453 when Mehmet II set his sights on the Kingdom of Hungary. The Ottoman forces were defeated by an army led by Catholic priest John of Capistrano and John Hunyadi. Crusade of Nicholas V (1455–1456).