The war was started by the leaders of Germany and Austria-Hungary. Vienna seized the opportunity presented by the assassination of the archduke to attempt to destroy its Balkan rival Serbia
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The simplest answer is that the immediate cause was the assassination of Franz Ferdinand, the archduke of Austria-Hungary. His death at the hands of Gavrilo Princip – a Serbian nationalist with ties to the secretive military group known as the Black Hand – propelled the major European military powers towards war.
