Final answer:
Truman was concerned about the events on the Korean Peninsula due to the use of subversive tactics to spread communism, the threat it posed to Taiwan, the defiance of UN resolutions by North Korea, and the overall potential for instability in the Pacific.
Step-by-step explanation:
President Truman had several concerns regarding the events on the Korean Peninsula. Firstly, the communists' use of subversive tactics was alarming, as it suggested a strategy to expand Communist influence underhandly. Secondly, the use of armed forces to spread communism, demonstrated by the North Korean invasion of South Korea, posed a direct challenge to international peace and the containment policy. Thirdly, the threat of a communist invasion of Taiwan (Formosa) was a significant concern for US foreign policy in Asia. The potential for instability in the Pacific area worried Truman as it could lead to wider conflicts and undermine the security of critical regions like Japan. Lastly, North Korea's defiance of UN orders reflected a disregard for international norms and heightened the potential for conflict.