Another reading from The Twilight of the Presidency:

Newton’s first law holds that a body at rest will remain at rest and a body in motion will continue in motion along the same line until acted upon by an outside force. The federal bureaucracy is like that. It can be moved by a determined president. But once the motion has started and the course has been set, the internal momentum of our government’s machinery continues with the implacable determination of an advancing glacier. It also dedicates its powerful capabilities to reinforcing the presidential belief that the original decision was correct. In a very real sense, the machinery has mechanisms that are potent in heading off any desire to change course. […] Whatever argument [the president] gets is unlikely to come from the internal structure of government, and when it does come that way, it is easy for him to  close his ears to it. He may get an occasional argument on how best to carry out a policy; but  rarely, if ever, that the policy he has set should be abandoned.