Truman was afraid that it could escalate into a war between the United States and the Soviet Union. In other words, he thought it might be the beginning of World War III. Hope this helped. :)
Truman reacted strongly to North Korea's attack on South Korea to prevent the spread of Communism and uphold democracy in the face of a perceived threat. His response was part of a larger strategy to contain Communism during the Cold War. Therefore, the correct answer is B: He wanted to prevent the advance of Communism.
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