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–Russia relations Treaty of Berlin. While Russia eventually backed down, relations between the two Empires were permanently damaged. The lasting result was bitter enmity between Austria-Hungary on one side and Serbia and Russia on the other. Following the assassination of Archduke Franz Ferdinand of Austria by Serb nationalists of the Black Hand secret society on 28 June 1914, Austria delivered the July Ultimatum to Serbia demanding that the Austrian police and military have the right to enter Serbia. Serbia rejected the ultimatum and on 28 July 1914, Austria-Hungary declared war on Serbia. On August 6, the Emperor Franz Joseph signed the Austro-Hungarian