President Truman fired General MacArthur over a combination of policy differences and insubordination. By mid-1951, Truman was working to open negotiations with China, a task made more difficult when MacArthur issued unauthorized statements regarding the expansion of the war into Manchuria.
The Korean War was a limited conflict between the United Nations and a North Korean-Chinese alliance. The immediate war aims were to safeguard the Republic of Korea against Communist aggression, rather than overthrowing the governments of the enemy states or occupying territory. General MacArthur publicly disagreed with the war's limited aims and the President's seeming unwillingness to expand the war. In addition to interfering with official diplomacy by making veiled threats of an invasion, MacArthur opened a line of communication with at least one member of Congress, thus subverting the normal chain of command.
On March 23, 1951, MacArthur circulated a memo containing his opinions regarding the practicality of forcing the defeat of the Chinese army in Korea. The next day, he authorized General Ridgway to advance across the 38th parallel without the authorization of either the President, the Joint Chiefs or the United Nations. Truman later cited this infraction as the immediate cause of the General MacArthur's termination.