As a former Army officer, I consider obeying my oath one of the things I *must* do. That oath says I must support the Constitution. The Constitution says, as a poster has pointed out previously, that Congress has the authority to dictate the regulations under which the military operates. So, yes, were I still on active duty, I would consider upholding the oath as something I *must* do.
But there are other duties in play here. See my post 172.