Final answer:
The US responded to major international issues like World War II, the Cold War, and the Vietnam War by adapting its foreign policy strategies.
Explanation:
The three major international issues during the 20th century that significantly impacted US foreign policy were World War II, the Cold War, and the Vietnam War. After World War II, the US shifted from isolationism to becoming a global superpower, leading to increased involvement in international affairs. During the Cold War, policies like Truman's containment doctrine and the Vietnam War shaped US actions, reflecting the dynamics of the time.
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