Truman fired MacArthur due to insubordination as MacArthur's desired actions conflicted with Truman's decisions.
The basic cause for Truman firing MacArthur was MacArthur's insubordination to his commander in chief. MacArthur wanted to expand the conflict into China, proposing actions that Truman disagreed with, leading to a public questioning of Truman's decisions. Truman viewed MacArthur's actions as violating the principle of civilian control of the military and a threat to escalating a war that could involve Soviet intervention.