Correct answer: the establishment of the League of Nations.
In a speech delivered to Congress in January, 1918, US President Woodrow Wilson set forth 14 Points that were his proposals for how to end the Great War (World War I) and establish international peace.
The League of Nations was the signature idea of US President Woodrow Wilson, point #14 of his 14 Points. He had recommended an international peacekeeping association which he recommended for post-war settlements. The Treaty of Versailles adopted that idea, but back home in the United States, there was not support for involving America in any association that could diminish US sovereignty over its own affairs or involve the US again in wars beyond those pertinent to the United States' own national security.