The Macedonian front, also known as the Salonica front (after Thessaloniki), was a military theatre of World War I formed as a result of an attempt by the Allied Powers to aid Serbia, in the autumn of 1915, against the combined attack of Germany, Austria-Hungary and Bulgaria. The expedition came too late and with insufficient force to prevent the fall of Serbia and was complicated by the internal political crisis in Greece (the National Schism). Eventually, a stable front was established, running from the Albanian Adriatic coast to the Struma River, pitting a multinational Allied force against the Bulgarian army, which was at various times bolstered with smaller units from the other Central Powers. The Macedonian front remained stable, despite local actions, until the Allied offensive in September 1918 resulted in Bulgaria capitulating and the liberation of Serbia.
Following the assassination of the Crown Prince by a Bosnian Serb, Austria-Hungary had attacked Serbia in August 1914 but had failed to overcome Serbian resistance. After the entry of the Ottoman Empire into the war on the side of the Central Powers (November 1914), the decisive factor in the Balkans became the attitude of Bulgaria. Bulgaria occupied a strategically important position on the Serbian flank, and its intervention on either side of the belligerents would be decisive. Bulgaria and Serbia had fought each other twice in the previous thirty years: in the Serbo-Bulgarian War of 1885 and the Second Balkan War of 1913. Bulgaria had suffered defeat in 1913, and the Bulgarian government and people generally felt that Serbia had stolen land which rightfully belonged to Bulgaria. While the Allies could only offer Bulgaria small territorial concessions from Serbia and neutral Greece, the Central Powers' promises appeared far more enticing, offering to cede most of the land Bulgaria claimed. With the Allied defeats at the Battle of Gallipoli (April 1915 to January 1916) and the Russian defeat at Gorlice-Tarnów (May to September 1915) demonstrating the Central Powers' strength, King Ferdinand signed a treaty with Germany and on 21 September 1915 Bulgaria began mobilizing for war.