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.
The current President is lawful, beyond doubt. And he will remain so until some Constitutional challenge is heard and ruled upon. A bunch of Freepers, birthers, Shriners, or Rotarians clammering that he is not doesn't count, even if they are right.