Annexation of Bosnia by Austrian-Hungary in 1908 to the detriment of Serbia.
Murder of Archduke Franz-Ferdinand, Sarajevo
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.
Austrian-Hungarian Empire invades Serbia
Austria-Hungary uses the assassination as an excuse for declaring war to Serbia. Backed by Germany, the Empire invades Serbia.
Germany declares war on Russia
Germany invades Luxembourg
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.
Germany declares war on France
The Belgian government refuses ; Germany declares war on France.