Harry Truman wanted to keep the Korean War small, MacArthur, after the U.S beat back the North Koreans to the original border line of the two countries, wanted to invade further. Harry Truman was worried about Chinese intervention, which happened when they invaded North Korea causing more causalities for the American Soldiers.
Not only this, but he actually thought the U.S should bomb china and have Nationalist Taiwanese attack China, because of these actions, he was relieved.