Criticism and disobedience--MacArthur actively ignored Truman and was known to criticize his ability as "Commander and Chief".
Truman wanted a careful war in Korea not wanting to provoke China or the USSR. Aggressive moves by MacArthur will eventually bring China into the Korean War. Truman will fire MacArthur for not following orders. MacArthur supported the use of atomic bombs against China and wanted to use the full power of the US arsenal.
