- After multiple assassination attempts on Archduke Franz Ferdinand of Austria and his wife Sophie in Sarajevo, Bosnia and Herzegovina, the couple is finally assassinated by a Bosnian Serb nationalist, Gavrilo Princip.
- The Austro-Hungarian government issues a 48-hour ultimatum to Serbia, making demands that are believed to be intentionally unacceptable, setting the stage for a conflict between the two nations.
August 1st, 1914:
- Germany declares war on Russia after failed diplomatic efforts to resolve tensions between Austria-Hungary and Serbia.
- France, as Russia's ally, mobilizes its military in support of Russia.
- Germany responds to the French mobilization by declaring war on France.
- Germany invades Luxembourg and Belgium, prompting Britain to declare war on Germany.
- The United States declares neutrality, but tensions between the country and Germany escalate as German U-boats sink several merchant ships, leading to increased animosity.