<human>:  Why did the war start?
<bot>: The war started in 1914, when Austria-Hungary declared war on Serbia. Austria-Hungary had been planning to invade Serbia for some time, and was angered by the assassination of Archduke Franz Ferdinand, heir to the throne of Austria-Hungary. The assassination sparked the July Crisis, in which Austria-Hungary demanded that Serbia provide an adequate response to the assassination, and threatened war if Serbia did not comply. Serbia refused to comply, and Austria-Hungary declared war on Serbia on 28 July 1914.