Salonica campaigns: In the summer of 1915, Bulgaria entered World War I on the side of the Central Powers; in September, Bulgaria attacked Serbia. An Allied expeditionary force that landed at Salonica in an effort to aid Serbia attempted to join forces with the Serbians but was thwarted by the Bulgarian victory at Babuna pass. The Allies retreated to the vicinity of Salonica. Meanwhile the Greek government under Venizelos, which had decided to support the Allies, fell when it was repudiated by King Constantine I. The Allies fostered the establishment at Salonica of a rival Greek government, under Venizelos, which declared war on the Central Powers. After the Allies began an invasion of Greece, Constantine abdicated (June, 1917) and Greece formally joined the Allies. A number of unsuccessful Allied campaigns were launched against the German and Bulgarian forces. Finally, in Sept., 1918, a new offensive was launched, and the Allies advanced northward along the entire front. Bulgaria capitulated on Sept. 30, Serbia was recovered by Nov. 1, and on Nov. 10 Romania was captured. The armistice of Nov. 11, 1918, ended the campaign.