On the 28th June, 1914, Archduke Franz Ferdinand and Duchess Sophie von Chotkova were assassinated by Gavrilo Princip. Several members of the Black Hand group in Sarajevo were arrested and interrogated by the Austrian authorities. The Austro-Hungarian government soon discovered that three men in the Serbian Army living in Belgrade had organized the plot.
On 23rd July, 1914, the Austro-Hungarian government demanded that the Serbian government arrested these three men and send them to face trial in Vienna.
On 25th July, 1914, Nikola Pasic told the Austro-Hungarian government that he was unable to hand over these three men as it "would be a violation of Serbia's Constitution and criminal in law" and appealed to Russia for help.
On 28th July, 1914, Austro-Hungarian declared war on Serbia.