28 June 1914,
In Sarajevo, Bosnia, a murder was committed which caused the First World War. Austrian archduke Francisco Fernando and his wife Sophie Chot shot dead Serbian nationalists Gavrilo Principe. The attack gave the excuse to the Austro-Hungarian empire to fight Serbia and stop its expansion. The Austro-Hungarians sought Germany’s alliance as they hoped that Russia would help their ally Serbia. The precarious European situation then broke out and the First World War began.
Source:
Josep Fontana, The Century of Revolution. A history of the world since 1914 (Ed. Criticism).