Serbia was unfinished business for the Central Powers after the abortive invasion of 1914. Austria-Hungary, fighting on two fronts after Italy’s entry to the war, had been reluctant to attack Serbia again. But the crushing victory over Russia at Gorlice-Tarnow relieved their northern front, and then, in September 1915, Bulgaria elected to enter the war on the side of the Central Powers with the specific objective of claiming Serbian territory. The Austro- German invasion began on 7 October, with the Bulgarians following on 14 October. The Serbian army had been decimated in the 1914 invasion, and was badly underequipped; two Franco-British divisions were sent from Greece to reinforce the Serbs, but arrived too late to influence the outcome. The Austro-German armies captured Belgrade on 9 October then swept south, joining the Bulgarians approaching from the east. By 29 November, the remnants of the Serbian army were huddled along their southwest borders.
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