There is a traditional explanation of this, that America didn't join the League of Nations because it was 'isolationist'. This is the simplistic view that you will find in most textbooks.
You can add to this more specific knowledge about the political battle between Wilson and his opponents, which ended in the rejection of the Treaty by the Senate.
You also need to know, however, that this is a very old-fashioned view of events, and the modern view of historians say that neither Americans not the Senate were really isolationist AT ALL, and that the Treaty was lost rather by Wilson's stupidity.
The American people had not wanted to go into World War One - America did not join in until 1917 - and when the war ended they rejected the Treaty of Versailles and the League of Nations. This is called 'isolationism' - the desire to keep out of foreign affairs.
American people were isolationist because [IMAGE]:
America regarded itself as the 'New World' and did not want anything to do with the 'Old World', which they saw as being corrupt, old-fashioned and full of dangerous ideas like Communism. When Wilson went to the Versailles Conference, he was the first US President EVER to visit Europe. Most Americans liked the Monroe Doctrine of 1823, that America should stay out of Europe's affairs, and Europe should stay out of America's.
American businessmen were worried about the COST of the League - paying taxes to pay for its organisation, and losing trade if it decided to impose sanctions.
c. American soldiers:
100,000 soldiers had died in the First World War, and many Americans couldn't see why American soldiers should die keeping peace elsewhere in the world.
d. German immigrants:
Many Americans were immigrants from Europe and they still had ties there. So German immigrants HATED the Treaty of Versailles just as much as the Germans in Germany. (Also, many Irish immigrants HATED Britain so much they didn't want to have anything to do with a League of Nations with the British in it).
The American colonies had once been part of an empire, but the American revolution was about freedom from empire. The Treaty of Versailles hadn't abolished the British Empires (indeed, it had added Mandates to them), and many Americans did not want to be part of a Treaty or a League with upheld the British Empire.
We are not internationalists, we are American nationalists.
Theodore Roosevelt, speaking in 1919
Roosevelt was a former President of the US.
Senator Borah's speech
We have entangled ourselves with all European concerns … dabbling in their affairs. In other words, we have surrendered, once and for all, the great policy of "no entangling alliances" upon which this Republic has been founded for 150 years.
[Acting according to the decisions of a League] is in conflict with the right of our people to govern themselves free from all restraint of foreign powers....
A real republic can not commingle with the discordant and destructive forces of the Old World. You can not yoke a government of liberty to a government whose first law is that of force. India, sweltering in ignorance and burdened with inhuman taxes after more than one hundred years of dominant rule; Egypt, trapped and robbed of her birthright; Ireland, with 700 years of sacrifice for independence – this is the atmosphere in and under which we are to keep alive our belief in democracy.
Senator Borah (19 November 1919).
Borah, a Republican Senator and isolationist, was speaking in the Senate debate abut the Treaty. India, Egypt and Ireland were in the British Empire
Isolationism/ Money/ American soldiers/ German immigrants/ Empire - can you see any of these prejudices influencing Senator Borah's speech in Source B?