Battle of Kolubara their respective commands, and the 5th and 6th armies were merged into a single 5th Army of 95,000 men. On 28 June 1914, 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 on suspicion that the assassination had been planned in Belgrade. The Austro-Hungarian government made the ultimatum intentionally unacceptable to Serbia, and it was indeed rejected. The Austro-Hungarians declared war on Serbia on 28 July and that same day the Serbs destroyed all bridges on the Sava
Austria–Serbia relations of Serbian revolts during the 1848 Revolutions were not opposed by the Habsburg rulers. Serbian claims were not recognized by Hungary was eventually placated with the Austro-Hungarian Compromise of 1867, further angering Serbian nationalists. One notable flare-up between the two countries was the 1906-1909 economic conflict known as the Pig War which occurred simultaneously with the diplomatic and military crisis over the Austrian annexation of Bosnia. Ultimately, the tensions between the two countries could not withstand the strain of Assassination of Archduke Franz Ferdinand of Austria by Bosnian Serbs in 1914. Following the July Crisis, Austria invaded Serbia to punish