Final answer:
Truman rejected MacArthur's plan due to fears of starting a global war with China, risking Soviet involvement, and undermining civilian control of the military. The president preferred a limited war strategy and consequently relieved MacArthur for insubordination.
Step-by-step explanation:
President Truman rejected General MacArthur's plan to invade China during the Korean War because he feared it would escalate into another global war. Truman was concerned about the possibility of the Soviet Union entering the conflict, as China and the Soviet Union had a mutual assistance pact, potentially leading to World War III.
Despite MacArthur being seen as a hero, Truman's cautious approach was driven by fears of a broader war and the potential involvement of nuclear weapons, as MacArthur had suggested. Ultimately, MacArthur was relieved of command due to his insubordination and for undermining the principle of civilian oversight of the military.