Answering this questions is largely a matter of conjecture because the United States did not ratify the Versailles Treaty (1919), and it did not join the League of Nations (1920). The key question is: Could World War II (1939–1945) have been avoided if the US had done the opposite?
In 1945, WWII ended and the United Nations was founded. Fortunately, there has not been a World War III. The UN's supporters take some of the credit for that. It is tempting to believe, therefore, that WWII could have been avoided had the US played an active role in the UN's predecessor, the League of Nations.
There are two problems with this supposition, though. First, the US has not been a particularly enthusiastic member of the UN. Second, one could argue that WWIII has been averted because the great powers possess nuclear weapons, and they realize that there would be no winners in a WWIII.
American participation in WWI was an aberration of sorts. It had never before participated in a land war in Europe. After the war, there was a strong public desire to return to a policy of relative isolationism. President Woodrow Wilson's political opponents in the US Senate understood this, and they took advantage of this sentiment to thwart Wilson's idealistic postwar goals.