Final answer:
Congress rejected the Treaty of Versailles and the League of Nations primarily due to concerns over sovereignty and avoiding international entanglements without congressional consent. The U.S. remained technically at war with Germany until 1921, and the absence of U.S. participation in the League of Nations weakened its authority.
Step-by-step explanation:
One of the primary reasons Congress rejected the Treaty of Versailles was the fear among Republicans and conservative Democrats that joining the League of Nations would compromise U.S. sovereignty. Another concern was that the U.S. would get drawn into international disputes without congressional approval for war. The treaty's Article X, which required member nations to stand against aggression, was particularly contentious, as it was seen as surrendering congressional power to declare war. To gain sufficient support, amendments called 'reservations' were introduced to the treaty, but even with these changes, the treaty was not ratified.
The rejection of the Treaty of Versailles resulted in the U.S. technically remaining at war with Germany until Congress passed the Knox-Porter Resolution in July 1921. This refusal to participate also damaged the international authority of the League of Nations, since the absence of U.S. participation weakened the organization's ability to resolve disputes and mediate conflicts effectively.