Bulgaria was still resentful after its defeat in July 1913 at the hands of Serbia, Greece and Romania. It signed a treaty of defensive alliance with the Ottoman Empire on 19 August 1914. It was the last country to join the Central Powers, which Bulgaria did in October 1915 by declaring war on Serbia. It invaded Serbia in conjunction with German and Austro-Hungarian forces. Bulgaria held claims on the region of Vardar Macedonia then held by Serbia following the Balkan Wars of 1912–1913 and the Treaty of Bucharest (1913). As a condition of entering WW1 on the side of the Central Powers, Bulgaria was granted the right to reclaim that territory.