In April 1914, Arch-duke Franz Ferdinand of Austria-Hungary was assassinated, which set off a chain reaction resulting in Europe erupting into war. World War I pitted the Allies, Great Britain, France, and Russia, against the Central Powers, Germany, Austria-Hungary, and the Ottoman Empire. In 1914, the United States declared itself to be a neutral nation under the Neutrality Act of 1914. While Woodrow Wilson attempted to keep the United States out of the war, the US struggled to remain out of the conflict. Three factors contributed to the United States entering World War I in 1917:

Unrestricted Submarine Warfare
Propaganda Campaigns, and
The Zimmerman Telegram
As a result of the United States entering World War, the war shifted, and the Allies began winning many battles against the Central Powers. As a result, on November 11, 1918, an armistice was signed, which brought the fighting to a halt. Eventually, the Treaty of Versailles was signed in 1919, which ended World War I and laid the groundwork for World War II.