Once the Ottomans conquered Serbia, they got rid of the Serbian upper class. Most Serbs worked as farmers on land owned by Turks. They had to pay high taxes to the Turks. Some Serbs were forced to become Muslims. But the Serbs had their own laws in the Ottoman Empire through the millet system. During the 19th century Serbia gradually became independent. During World War I Austria conquered Serbia but lost the bigger war. Serbia joined other Balkan countries to form Yugoslavia. 