The United States has over 700 military bases around the world, with hundreds of thousands of personnel manning them. If a new President was elected who was a strict isolationist, how much could he do by himself to deconstruct this worldwide network? It would would seem to me he has much latitude under the Constitution, but what if the Congress was hostile to his agenda? What kind of treaty obligations do we have with various host countries (e.g. the big ones – Germany, Japan, South Korea, Italy)? Could these thwart his plan?
The President of the United States has executive authority to negotiate or withdraw the United States from treaties without seeking congressional approval. The Senate has authority only to ratify treaties.
President Bush exercised this authority when he announced the US was withdrawing from the 1972 Anti-Ballistic Missile Treaty in December 2001.
As for troop movements, the President is commander in chief so he can order them all to return home whenever he likes. Logisitcal problems aside, I don’t think there’s anything to stop him.