Archduke Franz Ferdinand of Austria was visiting the Bosnian capital Sarajevo. But then Serbian nationalist Gavrilo Princip shot the Archduke and his wife in support of Black Hand, a nationalist group. On July 28, Austria-Hungary declared war against Serbia.
The alliance system overcomplicated the conflict. On August 1-3, Germany, to support Austria-Hungary, declared war on Russia and its ally, France. When Germany invaded Belgium, Britain declared war on Germany and Austria-Hungary.