The Ottoman victory in Varna, followed by their victory in the Second Battle of Kosovo in 1448, deterred the European states from sending substantial military assistance to the Byzantines during the Ottoman conquest of Constantinople in 1453. Although Pius II officially declared a three-year crusade at the Council of Mantua to recapture Constantinople from the Ottomans, the leaders who promised 80,000 soldiers to it reneged on their commitment.[12] The Ottoman Empire was free, for several decades, from any further serious attempts to push it out of Europe.[2]