If he tells you to do it, you do it. You may ask for clarification or raise questions about your orders, but if the president says, "Just do what I told you," then you do it. There's no reasoned debate. You do what you're told. If you don't or if you carry it out grudgingly, expect to be replaced with the proper ass-chewing coming somewhere before that.

Not so, in the UK Forces, the order must first be considered lawful, if you refuse to obey on those grounds, you will face the consequences of your your actions but will be given every opportunity to argue your case. I know that to be the case as I did exactly that on two occasions and won both times.