Serbia was invaded by Austria-Hungary after Austria-Hungary placed a stringent ultimatum to the Serbian government demanding full compliance to an Austro-Hungarian investigation of complicity by the Serbian government in the assassination of Archduke Francis Ferdinand. Serbia agreed to most of Austria-Hungary's demands but because it did not fully comply, Austria-Hungary invaded.

Serbia had the diplomatic support of Russia, and both Serbia and Russia resented Austria-Hungary's absorption of Bosnia and Herzegovina that held a substantial Serb population. Serbia had expanded in size through its actions in the Balkan Wars of 1912–1913 when the Ottoman Empire's control in the Balkans collapsed.

During the war, Serbia justified the war as being the result of Austro-Hungarian imperialism towards Serbs and South Slavs, Serbia cooperated with Yugoslavists including the Yugoslav Committee who sought pan-South-Slav unification, particularly through liberating South Slavs from Austria-Hungary. In the Corfu Declaration in 1917, the Serbian government officially declared its intention to form a state of Yugoslavia.

The first two allied victories in the war were won by the Serbian army, on the mountains of Cer and Kolubara, in western Serbia. The Austro-Hungarian army was expelled from the country, suffering heavy losses. Serbia suffered great losses during the war, almost 50% of all men and around 30% of its entire population were killed.

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