Final answer:
The entry of the United States into World War I shifted its foreign policy from isolationism toward international intervention. This change was prompted by incidents like the Lusitania sinking and the Zimmerman Telegram. Post-WWI, the U.S. made attempts to lead globally but reverted to isolationism, influenced by the establishment and limitations of the League of Nations. Option b is the correct answer.
Step-by-step explanation:
The entry of the United States into World War I marked a significant shift in the nation's foreign policy. Prior to the war, under President Woodrow Wilson's administration, the United States was largely isolationist, focusing on domestic affairs and maintaining a stance of neutrality in international conflicts, especially the escalating war in Europe. The Lusitania incident, where German U-boats sank an ocean liner carrying American passengers, and the subsequent Zimmerman Telegram, in which Germany attempted to entice Mexico into an alliance against the United States, forced Wilson's hand, culminating in the United States' departure from isolationism towards international intervention to protect its interests and support the Allied Powers.
The aftermath of WWI saw the U.S. attempt to take a leading role in international politics, most notably with President Wilson's advocacy for the League of Nations, an early attempt at collective security. However, the U.S. ultimately adopted a policy of international intervention. This trend continued post-World War I, and while the 1920s and 1930s saw a return to more isolationist sentiment within the United States, the foundation for a policy of intervention had been laid. The League of Nations, which the U.S. never joined, ironically played a part in the nation's return to isolationism, as it became a symbol of international entanglements that many Americans wished to avoid.