That's the definition of a civilian, not a commissioned officer. He is obliged to comply with all lawful orders. The orders committing U.S. Forces to combat in Iraq and Afghanistan were issued by President George W. Bush and were authorized by Congress. The Secretary of Defense and the Service Secretaries have lawful authority to order the deployment of individuals and forces pursuant to those orders. There is a reason that they call them orders, not invitations.
Exactly. If I were to order him to afghanistan he would have to refuse as it would not be a lawful order as I have no authority to issue such an order.
ALL orders issued by anyone in the military derive from the Constitutional authority invested in the president as the c-in-c. If the president is not lawful (and the current (p)resident is not) then no orders are lawful.