It' snot the military getting involved in politics. It's one man demanding to know if the orders he has received are valid. You do not stop being a citizen when you sign up for the army. It is his Constitutional right to know.
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.