Ideally, of course, we should say that, and it should be enforced by the barrel of a gun backed up by a badge.
But the reality is, that ship has sailed. They already know they're breaking the law, and they already know that nobody in authority is willing to do squat about it.
So now what? We will either be overrun, or people will have to do what is necessary without the authority of a badge...
Or, unicorns will dance across rainbows in the sky as the federal government uses its authority to treat illegal aliens like the lawbreakers they are.
Those are our three options.