While visiting Sarajevo, Bosnia and Herzegovina, Archduke Franz Ferdinand and wife are assassinated by a Serb.
Austria wanted to punish Serbia and decide to give Serbia demands. Although the Serbian leaders agreed to most of the demands, Austria did not want to negotiate.
The same day, Austria declares war on Serbia. Because of the multiple alliances, Austria declaring war begins the start of many wars and countries entering into the war. Russia, an ally on Serbia, moves their troops to the border of Austria.
The fact that the Austrians knew that the Slavs/Serbs were already upset with them ruling made the assassination even worse.