The assassination of Archduke Franz Ferdinand, heir to the throne of Austria-Hungary, by Yugoslav nationalist Gavrilo Princip in Sarajevo on June 28, 1914 set off a diplomatic crisis beween Austria-Hungary and Serbia. Ultimately, entangled international alliances formed over the previous decades were invoked. On July 28, 1914, the Austro-Hungarians declared war on Serbia, starting World War I.

On April 6, 1917, the U.S. joined its allies--Britain, France, and Russia--to fight in World War I. Under the command of Major General John J. Pershing, more than 2 million U.S. soldiers fought on battlefields in France.

More Info: www.wikipedia.org