July 28, 1914:
- Gavrilo Princip, a Bosnian Serb nationalist, assassinates Archduke Franz Ferdinand and his wife Sophie in Sarajevo, the provincial capital of Bosnia-Herzegovina, which was annexed by Austria-Hungary in 1908.
July 29, 1914:
- The Austrian government blames the Serbian government for supporting the assassination plot and issues an ultimatum containing harsh demands, expecting Serbia to reject them.
July 30, 1914:
- Serbia responds to the ultimatum, accepting most but not all of its demands. The Austrian government considers the response unsatisfactory and severs diplomatic relations with Serbia.
July 31, 1914:
- Following Germany's assurances of support, Austria-Hungary issues a formal declaration of war against Serbia, citing the need to punish Serbia for the assassination and secure Austrian interests in the Balkans.
August 1, 1914:
- In accordance with a defensive alliance with Serbia, Russia announces a general mobilization of its military forces. France, an ally of Russia, issues its own mobilization order.
August 2, 1914:
- In violation of international treaties and Belgian neutrality, German forces invade Luxembourg and march into Belgium in an attempt to outflank the French defensive line and seize strategic points, such as Liege and Namur.
August 3, 1914:
- Germany declares war on France, and France declares war on Germany. As Belgium's guarantor, Britain demands the withdrawal of German troops from Belgian soil. When Germany refuses, Britain declares war on Germany at 11:00 a.m.
August 4, 1914:
- German armies invade neutral Belgium, and Britain enters the war, marking the official start of World War I.