The Second Battle of Kosovo was the culmination of a Hungarian offensive to avenge the defeat at Varna four years earlier. In the three-day battle the Ottoman army under the command of Sultan Murad II defeated the Crusader army of regent John Hunyadi. After that battle, the path was clear for the Turks to conquer Serbia and the other Balkan States, it also ended any hopes of saving Constantinople. The Hungarian kingdom no longer had the military and financial resources to mount an offensive against the Ottomans. With the end of the half-century-long Crusader threat to their European frontier, Murad's son Mehmed II was free to lay siege to Constantinople in 1453.