World War I Germany's unfettered submarine warfare against American ships during World War I provoked the U.S. into abandoning the neutrality it had upheld for so many years. The country's resultant participation in World War I against the Central Powers marked its first major departure from isolationist policy.
The 20th Century: The End of US Isolationism. Against the recommendation of President Woodrow Wilson, the U.S. Senate rejected the war-ending Treaty of Versailles, because it would have required the U.S. to join the League of Nations.
