As the Turkish incursion into Europe started, Croatia once again became a border area between two major forces in the Balkans. Croatian military troops fought in many battles under command of Italian Franciscan priest fra John Capistrano, the Hungarian Generalissimo John Hunyadi, and Hungarian King Matthias Corvinus, like in the Hunyadi's long campaign (1443–1444), battle of Varna (1444), second battle of Kosovo (1448), and contributed to the Christian victories over the Ottomans in the siege of Belgrade (1456) and Siege of Jajce (1463). At the time they suffered a major defeat in the battle of Krbava field (Lika, Croatia) in 1493 and gradually lost increasing amounts of territory to the Ottoman Empire. Pope Leo X called Croatia the "forefront of Christianity (Antemurale Christianitatis)" in 1519, given that several Croatian soldiers made significant contributions to the struggle against the Ottoman Turks. Among them there were ban Petar Berislavić who won a victory at Dubica on the Una river in 1513, the captain of Senj and prince of Klis Petar Kružić, who defended the Klis Fortress for almost 25 years, captain Nikola Jurišić who deterred by a magnitude larger Turkish force on their way to Vienna in 1532, or ban Nikola Šubić Zrinski who helped save Pest from occupation in 1542 and fought in the Battle of Szigetvar in 1566. During the Ottoman conquest tens of thousands of Croats were taken in Turkey, where they became slaves.