At the beginning of the First World War, Bulgaria pursued a policy of neutrality. Due to Bulgaria's strategic position in the east, it was pressured by both the Allies and the Central Powers to join the war. On September 6, 1915, Bulgaria affiliated itself with the Central Powers. On October 14, Bulgaria declared war on Serbia and invaded. By the end of November the Central Powers controlled all of Serbia. In 1916, when Romania entered the war on the side of the Allies, Bulgaria, together with the other Central Powers, overran Romania, and Bulgaria acquired Dobruja from Romania. In September 1918 the Allies broke through in Macedonia and Bulgaria was forced to capitulate. Under the Treaty of Neuilly, signed the next year, Bulgaria was forced to cede West Thrace and a part of Macedonia and return Dobruja to Romania.