Austria-Hungary starts the bombardment of Belgrade. What follows is a deployment race of armies between all major powers in Europe. Nobody wants to be unprepared in case of an attack.
Germany is implementing the Schlieffen-Plan. The idea is to avoid a two front war by conquering Paris via Belgium. After that the full attention would be diverted to Russia. One thing gets clear in the first days at the Western Front: This war is going to be different - this was modern warfare.