Final answer:
Truman rejected MacArthur's plan to expand the Korean War into China to avoid a broader global conflict, maintain civilian control of the military, and prevent acts seen as insubordinate, which General MacArthur's public criticisms and unauthorized actions represented.
Step-by-step explanation:
President Truman rejected General MacArthur's plan to expand the Korean conflict into China because of significant concerns about escalating into a larger, potentially global conflict. MacArthur's proposal included blockading China's ports and bombing industrial centers, which Truman feared could provoke Chinese and Soviet retaliation, potentially leading to World War III.
The key issues with MacArthur's approach were that it threatened civilian control of the military, risked broadening the war, and could have led to confrontation with the Soviet Union due to its mutual assistance pact with China.
Despite MacArthur's popularity and status as a military hero, Truman upheld the principle of civilian control over the military and had to act against what he saw as insubordination. MacArthur's dismissal came after he publicly criticized the president and bypassed protocols with unauthorized diplomatic communications, which were acts Truman and his advisers saw as dangerously close to treason.
Truman, prioritizing a limited war strategy, replaced MacArthur with General Matthew Ridgeway, who managed to push North Korean forces back to the 38th Parallel and secured South Korea, allowing Truman to seek an armistice and stabilize the situation without further escalation.