On October 9, 1915, Austro-Hungarian forces capture the Serbian capital of Belgrade, assisted in their defeat of Serbian forces by German troops under the command of General August von Mackensen.
It was not the first time during World War I that Austrian troops had occupied Belgrade. They had captured the city on December 1, 1914, effectively accomplishing what might have been their foremost war-making objective the previous summer: bringing the upstart Serbia to its knees after a Bosnian Serb nationalist shot and killed Archduke Franz Ferdinand of Austria during an official visit to Sarajevo, the Bosnian capital. As the American war correspondent John Reed observed during his visit to Serbia that winter, Austro-Hungarian forces reduced many areas of Belgrade to ruins, including its university: "The Austrians had made it their special target, for there had been the hotbed of pan-Serbian propaganda, and among the students that formed the secret society whose members murdered the Archduke Franz Ferdinand." Two weeks after the Austrians entered Belgrade, however, Serbian forces recaptured the city, taking 40,000 Austrian prisoners.
By the autumn of 1915, though, Serbia’s prospects were dwindling, and at the end of September, the Austrian army—composed of both Austrian and German troops—stood to the northwest and north of Belgrade, reinforced by the German 11th Army nearby. On October 6, German and Austro-Hungarian troops under the command of General von Mackensen crossed the wide Danube River in heavy rains, closing in on Belgrade. Three days later, they entered and took control of the city, forcing the Serbs to evacuate.
Though the Serbs planned to counterattack, their defeat was sealed only days later by the entrance into the war of Bulgaria, whose forces immediately invaded Serbia and Macedonia, the former Ottoman province in the Balkans it had long coveted. Bulgaria’s expressed reason for joining the Central Powers—aside from its economic relationships with Germany and Austria—was to annex Serbian territory. Its army neatly closed Serbian forces off from its allies, including a British and French force newly arrived in Greece for the purpose of aiding the Serbs. By the end of November, both Serbia and Macedonia were in the hands of the Central Powers.
Of all the belligerent nations during World War I, Serbia suffered the greatest number of casualties in relation to the size of its population. Its losses were staggering: Of some 420,000 soldiers in September 1915, 94,000 were killed in action and another 174,000 were captured or missing, while undoubtedly great numbers of civilian casualties remained uncalculated.