On 28th June 1914, the heir to the Austro-Hungarian throne, Archduke Franz Ferdinand and his wife, Sophie were assassinated during a state visit to Sarajevo. Their murderer, Gavrilo Princip, although born in the Empire of Austria-Hungary, was also a Serb. Austria-Hungary, therefore, used the assassination as an excuse to issue the Serbian government with a demand that a full investigation should be carried out. Serbia denied responsibility for the assassination and refused to comply. Austria-Hungary, keen to dispose of the perceived Serbian threat once and for all, then approached her ally, Germany, to seek support. Germany, in fact, offered a ‘blank cheque’, stating that she would back her neighbour in any action against Serbia. Both countries knew that Russia would support the Serbs, so a show of joint strength was essential.
Austria-Hungary then issued the Serbian government with a series of demands, knowing that some of these would be unacceptable. Serbia, having received support from Russia, rejected the ultimatum. On 28th July 1914, Austria-Hungary, therefore, took the next step and declared war on Serbia. From this moment on, events began to move on quickly. Russia announced the mobilisation of her vast army, forcing Germany, who regarded this as a direct threat against her ally, to declare war on Russia on 1st August. France, bound by its treaty with Russia, was also drawn into the conflict.