Final answer:
Isolationism delayed U.S. entry into WWI as the policy favored avoiding international conflicts. After WWI, U.S. rejection of the League of Nations reinforced isolationist sentiment. The interwar period saw the United States adopting Neutrality Acts to maintain isolationist policies.
Step-by-step explanation:
The effect of isolationism on U.S. entry into World War I was that it delayed U.S. entry into the war. Isolationism was a foreign policy adopted by the United States that aimed to keep the country out of international conflicts unless there was a moral imperative to intervene. This policy was reflected in the United States' reluctance to enter WWI until several events, such as the sinking of U.S. ships by German U-boats and the Zimmerman Telegram, swayed public and political opinion. Eventually, strategic interests and the infringement of American neutrality made entry into the war seem necessary to a growing number of Americans.
After World War I, the establishment of the League of Nations was pivotal in reinforcing isolationist sentiment. President Wilson advocated for the League as a means to ensure collective security and peace, but the U.S. Congress rejected joining the League, largely due to concerns over entanglement in future European conflicts.
During the interwar years, isolationism influenced the enactment of a series of Neutrality Acts intended to limit U.S. involvement in international conflicts. These acts highlighted the strong desire of the American public to avoid entering into another war like WWI. However, with the rise of aggressive totalitarian regimes in Europe and Asia, isolationism was challenged as international tensions escalated prior to World War II.