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
or
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.
By having the Chorus spoil the end of the play before it even starts—Romeo and Juliet die, and only then do their families end the feud Shakespeare sets up similar possibilities for the development of irony, tension, and catharsis here.
