July 28, Noon: Austria declares war on Serbia. Other outbreaks play out over the next several months.
World War I (1914–1918).
The Entente Powers (initially France, Britain, Russia, and then joined later by Italy and the USA) v. The Central Powers (initially Germany, Austria–Hungary, and later joined by the Ottoman Empire and Bulgaria).
Ended by the Treaty of Versailles, which was then followed by the creation of the League of Nations.