The Balkans theatre, or Balkan campaign, of World War I was fought between the Central Powers and the Allies.
The campaign began in 1914 with three failed Austro-Hungarian's offensive into Serbia. A new attempt a year later by the combined forces of Austria-Hungary, Germany and Bulgaria led to the conquest and occupation of Serbia and Montenegro. The Serbian army did not surrender but retreated through the mountain of Albania and was evacuated to Corfu before reforming in Salonika a few months later. On the Macedonian front, the Royal Serbian Army joined the Franco-British Allied Army of the Orient and fought a protracted trench war against Bulgarian and German forces. The allied army presence in Greece resulted in the National Schism on whether Greece should join the Allies or remain neutral, which would benefit the Central Powers. Greece eventually joined the Allied Powers in 1917. In September 1918, the Vardar Offensive broke through the lines of Bulgaria, which was forced to surrender, leading to the liberation of Serbia, Albania and Montenegro.