Bulgaria, of all the Balkan states, was the only one to join the Central Powers, led by Germany and Austria, in World War I. It was deeply ironic that Bulgaria chose to side with her former enemy and oppressor, Turkey, and against her former friend and protector, Russia. Again, the issue for Bulgaria was the Macedonian question. Serbia and Greece, which had triumphed over Bulgaria in the Second Balkan War, were allied with the entente powers. Bulgaria chose to fight against these enemies in order to regain Macedonia. Although the entente powers hoped to woo Bulgaria to their side, they refused--because of Serb and Greek pressures--to cede Macedonia to Bulgaria. The Central Powers, on the other hand, who were already at war with Serbia, were willing to promise Macedonia to the Bulgarians in exchange for their collaboration. In the early stages of the war Germany won victories in France and on the eastern front. Although the government then ruling Bulgaria was already inclined to join the Central Powers, these early successes made German promises even more appealing. In August 1915 a secret treaty of alliance was signed by Bulgaria and Germany, containing a clause that promised Serbian, Greek, and Romanian territories to the Bulgarians. Thus the quadripartite alliance was born, composed of Germany, Austria-Hungary, Turkey, and Bulgaria. By September 1915 Bulgarian troops were mobilized and began to deploy along the borders of Greece and Serbia. On October 1, 1915, Bulgaria declared war on Serbia and, with the assistance of Austrian and German troops, succeeded in defeating the Serbian army. At the same time the Bulgarian army began to advance on Macedonia. There the local population, a proportion of which was openly sympathetic to Bulgarian aspirations, joined in the fighting on the side of the Bulgarians. Although the Bulgarian army attempted to drive the entente forces from southern Macedonia, it met with failure. This defeat was followed by a period of prolonged trench warfare on the Balkan front. By 1916 Bulgaria was also at war with Romania and, with the help of German and Austrian units, managed a victory over the Romanians.