C. Feared the spread of communism.
Step-by-step explanation:
Harry Truman got the United States involved in the Korean War because he thought that if nothing were to be done, communism would spread.
In Truman's declaration of war speech, he mentioned communism. This made it quite evident that he believed without an involvement of the United States, communism would probably spread quite rapidly. The Korean War hurt Korea's economy very badly and killed many of people, but finally a peace treaty was put out. This treaty also designated a wall to help prevent future wars.
Without the thinking of Harry Truman, things may have gotten out of hand with the spread of communism.