The assassination of the Austrian Archduke Franz Ferdinand in 1914 was the immediate cause of the First World War. For four years, the great European powers fought a gruesome battle.
Murder of the Austrian Archduke: Start of the First World War
June 28, 1914 Sarajevo
In response, Austria-Hungary set Serbia an ultimatum. If Serbia failed to meet its demands, the Austro-Hungarian army was going to invade Serbia. The Serbs accepted all demands, except one. They wanted to investigate the murder themselves, without interference from the Austro-Hungarian representatives. Austria-Hungary did not agree and on 28 July 1914 declared war on Serbia.
The fact that this crisis resulted in a world war was due in part to the alliances between the various European countries. As many states were bound by treaties to help one another in the event of war, there were two camps: the Central Powers (Austria-Hungary and Germany) and the Allied Powers (France, Russia and England).
Both sides expected the war to be over soon, but they were wrong. The First World War lasted four years, and millions of soldiers from all over the world were killed.