Peter Beinart nails it:
Truman didn’t just fire MacArthur because the general treated him with disrespect. He fired him because MacArthur wanted to do whatever it took to liberate the Korean peninsula, including bombing mainland China, whereas Truman came to realize that Korea must be a limited war, fought merely to preserve South Korean independence. In insisting that America’s Cold War strategy be the containment of communism, not the rollback of communism, Truman kept the pursuit of military victory from destroying American power.
In that sense, McChrystal's self-immolation is a big opportunity for the president. He needs to restate his commitment to a limited war that begins to wind down next year. He gave the COIN version of neo-imperialism its best shot and it shot itself in the foot. Now: leave.