The president has supreme authority over the executive branch, which includes the military. Trump has explicitly said that he would implement mass deportations, and has called democrats “the enemy within” while in the same breath saying that the national guard could be used to put them in prison.

As for who’s going to let it happen, that would be all of the Trump sychophants that he will be appointing. He learned last time that appointing republicans isn’t enough to let him get away with his batshit crazy ideas, they have to be cult loyalists as well.