The assassination of Archduke Franz Ferdinand, who would have inherited the throne of the Astro-Hungarian Empire, started World War I. He and his wife were killed by Gavrilo Pincip, a Serbian nationalist, on June 25, 1914.
Austria-Hungary wished to retaliate against Serbia for the assassination of Franz Ferdinand, but was not strong enough, so the country called on an alliance with Germany for help. While this alliance was solidifying, Serbia made an alliance with Russia. Since Russia had a treaty with both Britain and France, the two countries joined in. These treaties and alliances created a conflict across all of Europe instead of one between two countries. These two groups were called the Allied Forces and the Central Powers. Initially, the Allies included Russia, Britain and France, while the Central Powers were Germany and Austria-Hungary. As the war continued, other countries became involved.
The United States didn't get involved until April 6, 1917, after a message to Mexico from Germany fell into British hands. Around this time, Russia bowed out of the conflict due to internal problems. Other countries involved included Japan, Italy and areas of the Middle East. The war officially lasted from July 28, 1914 to Nov. 11, 1918 when the two sides signed an armistice.