Final answer:
The United States did not join the League of Nations primarily due to opposition from Congress.
Step-by-step explanation:
The primary reason why the United States did not join the League of Nations was due to opposition from Congress. The U.S. Senate, dominated by the Republican Party at the time, expressed concerns that joining the League would compromise their constitutional power to declare war and draw the country into foreign conflicts. They also objected to the control exerted by Britain and France over the League.