After fighting started in 1914, America chose to remain neutral, figuring the fighting was essentially Europe’s problem. The country
did attempt to play peacemaker, although that didn’t go so well. After a German U-boat sank the Lusitania in 1915, killing 128 Americans on board, public opinion in the United States started to turn firmly against Germany. President Wilson demanded Germany stop attacking passenger ships and once again tried to mediate a settlement between the Allied and Axis powers. The peace didn’t happen, and Wilson found himself coming under increasing fire from former president Theodore Roosevelt and his Secretary of State, William Jennings Bryan, who resigned because he could no longer support the president’s policy.
Two suspected acts of German sabotage—the Black Tom Explosion in Jersey City in July 1916 and the Kingsland explosion in Lyndhurst, New Jersey in January 1917 convinced most people that neutrality was no longer an option. In January 1917, Germany also resumed unrestricted submarine warfare, and the German Foreign minister sent the Zimmermann Telegram to
Mexico, inviting the country to join the war on Germany’s side. In return, Germany promised to help Mexico recover territories lost during the Mexican-American War. Americans were outraged. War was declared on April 6, 1917.
America mobilized quickly—far more quickly than Germany anticipated. The U.S. congress gave citizenship to Puerto Ricans when they were drafted in 1917, and by summer 1918 we were sending 10,000 fresh soldiers to France every day.
The war dragged on for more than a year, but the addition of American troops and new supplies definitely helped turn the tide. Throughout the fall of 1918 the Axis powers capitulated one by one, with Germany signing an armistice last on November 11. A formal state of war technically persisted for several months after the armistice, until the Treaty of Versailles was signed on June 28,1919.