Answer: Austria-Hungary declared war on Serbia.
Explanation:
In June 1914, Archduke Franz Ferdinand who was next in line to the throne of Austria, was killed along with his wife whilst they were touring Sarajevo in Bosnia-Herzegovina by a Serbian nationalist.
Tensions had already been brewing between Austria-Hungary and Serbia and when the Archduke was assassinated, the Austria-Hungary government sent ultimatums to Serbia that they had to abide by to avoid war. Serbia agreed to all but one of the provisions but a month later the Austrians declared war.
This dragged Germany into the conflict as well as Russia which then dragged France and then Britain in as well thereby starting, the First World War.
