American entry

The United States stayed neutral until 1917. American entry was triggered by Germany's resumption of unrestricted submarine warfare (which sank American ships) and by the Zimmermann Telegram, in which Germany proposed an alliance with Mexico against the United States. President Woodrow Wilson asked Congress for war in April 1917 with the stated goal of making the world "safe for democracy." American troops arrived in numbers in 1918 and helped tip the balance against an exhausted Germany.