They are not forbidden from determining that a given order is lawful or not. However they are trained that the presumption is that a given order is lawful unle

They are not forbidden from determining that a given order is lawful or not. However they are trained that the presumption is that a given order is lawful unless it's clearly contrary to the Constitution, law, regulation or policy. As we phrase it in the Navy, no sailor has ever been court-martialed for following a legitimate order, but the same cannot be said regarding mutiny. In short if you want to challenge an order go right ahead, but realize that your ass is going to be put on the line and even the Supreme Court will be biased on the military's side regarding it, if previous experience is any judge.