I think a drastic reduction in personnel/bases/equipment if it was done too quickly could be very destabilizing. While I don't like America policing the world, that is the current state of affairs and changing that abruptly could be dangerous.

Cutting expenses by 50% would also be a big change and likely affect some personnel/bases/equipment but also likely cut out a lot of waste and grift. I am not sure you could do it without affecting veterans benefits though which will be immensely politically unpopular.