Answer:
The league of nations was an idea created by President Woodrow Wilson (from the US obviously) after World War one in order to prevent a world war at that scale from occurring again.
At the end of the WW1 the Treaty of Versailles was created in order to end the war after Germany surrendered but US Congress never ratified the Treaty of Versailles and the US never joined the league of nations because it was part of the Treaty of Versailles.
Many historians believe that if America had joined the League, there would have been a lot more support in preventing conflicts.
The League of Nations was formed to prevent a repetition of the First World War, but within two decades this effort failed. Economic depression, renewed nationalism, weakened successor states, and feelings of humiliation (particularly in Germany) eventually contributed to World War II.
In the end The League of Nations was changed to the UN (United Nations) and many other countries joined, as well as the US.