Final answer:
The League of Nations formation showed the impact of nationalism on foreign policy after World War I.
Step-by-step explanation:
The League of Nations formation showed the impact of nationalism on foreign policy after World War I.
After the war, President Woodrow Wilson proposed the idea of collective security and the establishment of the League of Nations to prevent future conflicts. However, the United States eventually rejected membership in the League of Nations, partly due to a rise in isolationism and nationalism.
This decision to not join the League of Nations demonstrated a shift towards isolationism and a prioritization of national interests over international cooperation.