Do YOU think we're going to actually round up and deport 12 million? EACH person we round up gets a court hearing. That's 12 million court hearings. That will last from now until Admiral Kirk finally whacks Khan.
We used to be able to do these deportations but the courts have intervened and ruled they must be given hearings.
So, basically, we're f****D.
Plenty of space to hold them out west along the border until they get their turn. Sure is cheaper than providing medicaid, food stamps, educating their anchor babies, etc. Give them a choice: return now, of your own free will; or wait it out in Tent City with the roadrunners and rattlesnakes behind the wire, like any other lawbreaker. No teevee, no HVAC, diet taken from Michelle Obama's guidelines. Shower once a week. The harder they work on the border fence, the sooner higher the priority their case gets and they can go home.
And that is worst-case, "Takes forever to process them" assumptions. Fire (or re-located to rubber rooms) and re-staff with folks willing to do the job and it move more quickly. Start hauling off those who employ illegals to prison and demand for illegals will dry up quite a bit. Use every contact with gov't (LEO, school, indigent hospital care, etc.) to nab them. Truly make those who want to stay go "into the shadows" and stay there.
This is the least-bloody option.
It should be the last piece of libertarian policy enacted, not the first.
Open borders is the clearest evidence that libertarianism is a utopian faith. The moment it achieves its objectives, it is undone by the results of the achievement. "We have an-cap nirvana. Let us now open the borders to roughly 6 billion folks who do not share our political creed. What could possibly go wrong?"