Question

Why didn't the United States join the League of Nations?

Answer 1

When it came to the Congress to approve the joining of the United States in the League of Nations it was blocked by the Republican opposition, especially from Senators William Borah and Henry Cabot (D).  

The U. S. public opinion was still disappointed over the outcomes of the war. Also, the Republican Senators did not like what they thought to be a violation of the U.S. sovereignity: the covenant of the League in it's Article X predicted that in case of a member being attacked all the others should defend it.

This added to the historical isolationism of U.S. diplomacy stopped the country from joining the League of Nations despite its inspiration on President Woodrow Wilson’s Fourteen Points.