The League of Nations was Woodrow Wilson's idea - The American President. (14th point of his 14 points)
In 1919-1920 he took his idea to Congress twice but was defetaed both times.
1920 - He was too ill to run for president and so Congress rejected his idea of the League of Nations.
Thus, in 1920 when the League opened the American seat was empty - USA didn't join.
Reasons why America didn't join the League.
1. The Treaty of Versailles:
Many Americans hated it. German immigrants were also opposed to it ( they didn't even want the USA to join WWI). Therefore they didn't want the USA to be part of an organisiation that accepted…