Listened to a Radiolab podcast recently on the steps needed for the Americans to launch a nuclear weapon and it's surprisingly concerning the lack of checks and balances the process has on the President himself if he decides to go for it... While there are two guys responsible for "turning the switch", which provides checks and balances at that point, a launch command from the President cannot be negated by anyone and it is the President's decision alone.
http://www.radiolab.org/story/nukes/