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The Balkans theatre or Balkan campaign was a theatre of World War I fought between the Central Powers (Austria-Hungary, Bulgaria, Germany and the Ottoman Empire) and the Allies (Serbia, Montenegro, France, the United Kingdom, Russia, Italy, and later, Greece).

The offensive began in 1914 with three failed Austro-Hungarian offensives 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 military did not surrender, retreating through the mountains of Albania and 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 had broken through the Bulgarian lines, forcing them to surrender. Serbia, Albania and Montenegro 's liberation soon followed.