Final answer:
The best example of the expansion of the Truman Doctrine, which aimed at containing communism, is the U.S. entry into the Korean War, as it was a direct military action in support of a free nation against communist aggression.
Step-by-step explanation:
The Truman Doctrine was a principal strategy for the United States during the early years of the Cold War, aimed at containing the spread of Communism. It was articulated by President Harry S. Truman in 1947, pledging to provide support to nations threatened by Soviet communism. The best example of the expansion of the Truman Doctrine, which committed the United States to resist the spread of communism wherever it seemed to flare up, was the U.S. entry into the Korean War.
In 1950, after the North Korean communist regime invaded South Korea, the United States intervened militarily to prevent the fall of South Korea to communism. This action was in line with the Truman Doctrine's policy of containing communism and supporting free peoples against communist subjugation, making it a clear extension of those principles. Unlike detente, which aimed to ease tensions, or the arms race, which was about maintaining a balance of power, the intervention in Korea was a direct military action to counteract the influence of communism in line with Truman's policy of containment.
Final answer: C. entry into the Korean War. This represents the best example of the Truman Doctrine's expansion as it was an explicit act of military support to a free nation resisting communist takeover, consistent with Truman's commitment to contain communism.