Millions killed? Who's in "fantasy land" now? LOL ... If it were to come to conflict, I'd hire the mexican immigrants, even illegals, and train them as a military border force ... fight fire with fire! Heck it's working in Iraq like a charm.
This whole concept of mass deportation is a strawman IMHO. Enforce verifiable documentation. Make the employers stop hiring illegals, with the threat of fines over their heads. Over time, the market for illegal labor will end and the 'magnet' for illegal immigration will cease to operate.
A worst-case - but not unlikely - scenario.
Assuming your documentation/enforcement system worked...what would you do about a destabilized Mexico, about American lives and possession under attack in that country?