The purpose of enlistment is not an issue(?) The enforcement of immigration is not cut and dry. There could be situations when determining nationality could be placed or forced on the Guardsman. We have seen families separated, mothers from children.
I would tell my superior officer to go fcvk himself if he/she ordered me to do something like that. There are rules and obligations in the military that simply can't extend to civilian enforcement. This is why we have laws prohibiting military enforcement against citizens in the United States.
And Northern, knowing you right wingers love the idea of bringing violence and chaos whenever possible to as many as possible, when the ball starts rolling downhill it doesn't care how many trees it hits. When whatever enforcement agencies begin, they won't give a shit about rules.