Germany used unrestricted submarine warfare (sink everyone military-related)
In 1915, Germans sunk a British passenger ship, The Lusitania off the coast of Ireland which had 128 Americans traveling to Britain on it.
President Woodrow Wilson used the event as propaganda, which vilified Germans, and made many want to enlist.
Germany stopped using unrestricted submarine warfare
In 1917, the British intercepted a telegram from the German foreign minister, to his ambassador in Mexico.
America then officially entered WWI