Chinese Intervention: MacArthur's advance toward the Yalu River, which forms the border between North Korea and China, alarmed the Chinese leadership. They saw it as a direct threat to their own security and decided to intervene in the conflict. In October 1950, Chinese forces crossed the Yalu River and launched a massive counteroffensive, driving the United Nations (UN) forces back.
Stalemate and Protracted War: The Chinese intervention prevented the UN forces from achieving their objective of unifying Korea and led to a stalemate in the war. The fighting dragged on for several more years, resulting in heavy casualties and destruction on both sides. The war ultimately ended in a ceasefire agreement in 1953, leaving Korea divided along roughly the same lines that it had been before the war.
Escalation of Tensions with the Soviet Union: The U.S. government was unaware of China's plans to intervene in the Korean War, and MacArthur's advance toward the Yalu River caught them off guard. The surprise Chinese intervention raised concerns about the potential involvement of the Soviet Union, China's ally, and heightened tensions between the U.S. and the Soviet Union during the Cold War.
Dismissal of MacArthur: MacArthur's decision to advance toward the Yalu River was controversial within the U.S. government. President Truman was concerned that MacArthur's actions could lead to a wider war with China and possibly the Soviet Union. As a result, Truman relieved MacArthur of his command in April 1951, leading to a public uproar and further divisions within the U.S.
In summary, General MacArthur's decision to advance toward the Yalu River during the Korean War had far-reaching consequences, including the Chinese intervention, a protracted war, increased tensions with the Soviet Union, and the dismissal of MacArthur himself. These developments significantly altered the course and outcome of the Korean War.