Annexation of Bosnia by Austrian-Hungary in 1908 to the detriment of Serbia.
On 28 June 1914, in Sarajevo (capital of Bosnia-Herzegovina), the Serbian student Gavrilo Princip kills by bomb the heir to the Austrian-Hungarian throne, Archduke Franz-Ferdinand and his wife Sophie.
Austria-Hungary uses the assassination as an excuse for declaring war to Serbia. Backed by Germany, the Empire invades Serbia.
According to its alliance with Russia, France mobilises its troops. The German were convinced they could quickly attack and defeat France. Germany invades Luxembourg and asks neutral Belgium to lets its troops cross its territory so as to reach France.
The Belgian government refuses ; Germany declares war on France.