Which of these was one of President Truman's concerns during the Korean War?

how to prevent the war from spreading to Vietnam

how to keep the Soviet Union from attacking Japan

how to prevent a communist uprising in South Korea

how to keep China from entering on the side of North Korea

Truman was most concerned with the newly communistic country of China joining the war effort. The US and United Nations could handle North Korea and were able to push back forces out of South Korea. China entering meant more men, more money, and more arms. It also could eventually lead to an escalation of warring powers coming into the war. China did enter the war which led to an armistice and agreement to keep Korea split at the 38th parallel.