The assassination of Archduke Franz Ferdinand (the heir to the throne of the Austrian Empire) on 28th June 1914 was done by a Serb Nationalist who wanted Bosnia to become part of Serbia, and not be part of the Austrian Empire. Austria blamed the government of Serbia for organising the assassination. But Serbia had an alliance with Russia (and Russia had an alliance with Britain and France), and Austria had an alliance with Germany. So a small local dispute in the Balkans turned into a huge war across the continent.