The Battle of Kolubara (Колубарска битка, Schlacht an der Kolubara) was fought between Austria-Hungary and Serbia in November and December 1914, during the Serbian Campaign of 1914.
It commenced on 16 November, when the Austro-Hungarians under the command of Oskar Potiorek reached the Kolubara river during their third invasion of Serbia that year, having captured the strategic town of Valjevo and forced the Serbian army to undertake a series of retreats. The Serbs withdrew from Belgrade on 29–30 November, and the city soon fell under Austro-Hungarian control. On 2 December, the Serbian army launched a surprise counterattack all along the front. Valjevo and Užice were retaken by the Serbs on 8 December. The Austro-Hungarians retreated to Belgrade, which 5th Army commander Liborius Ritter von Frank deemed untenable. The Austro-Hungarians abandoned the city between 14 and 15 December and retreated into Austria-Hungary, allowing the Serbs to retake their capital the following day.
The Austro-Hungarians and the Serbs suffered heavy casualties, with more than 20,000 dead on each side. The defeat humiliated Austria-Hungary, which had hoped to occupy Serbia by the end of 1914. On 22 December, Potiorek and von Frank were relieved of their respective commands, and the 5th and 6th armies were merged into a single 5th Army of 95,000 men.
On 28 June 1914, the Bosnian Serb student Gavrilo Princip assassinated Archduke Franz Ferdinand of Austria in Sarajevo. The assassination precipitated the July Crisis, which led Austria-Hungary to issue an ultimatum to Serbia on 23 July because it suspected that the assassination had been planned in Belgrade. The Austro-Hungarian ultimatum was intentionally unacceptable to Serbia and was indeed rejected. The Austro-Hungarians declared war on Serbia on 28 July. The same day, the Serbs destroyed all bridges on the Sava and Danube rivers to prevent the Austro-Hungarians from using them during any future invasion. Belgrade was shelled the following day, which marked the beginning of World War I.