Answer:
Indeed, President Truman was very wise in participating in the Korean War. At the beginning of the Cold War, both sides were in the midst of preparatory movements and geopolitical positions that would be key to the future of the war. In this sense, a North Korean victory would have meant complete control over the continental east, and an almost certain fall of Japan into communist hands, which would have created a new axis between the USSR-China-Korea-Japan, extremely powerful and difficult to combat, which would have triggered an almost certain Third World War.
In addition, at that time South Korea was surely the poorest and underdeveloped area of the peninsula, so without any help its survival in the face of the communist attack would have been unlikely.
American participation served to contain communism, guaranteeing the development of Japan as an unconditional ally in the Pacific, in the face of the communist threat in the area.