It was because of the leaders of Germany and Austeria hungray. Vienna seized the opportunity to presented by the assination of the archduketo attempt to destroy its Balkan rival Serbia.
The assassination of Archduke Franz Ferdinand was the immediate cause of a war between two great powers—Russia and the Habsburg Austro-Hungarian Empire.
Explanation:
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