Oh, I agree, I'd just move the fortified border 200 miles South.
Hmmm. You'd have to displace a whole lot of Mexicans, turning them into the Palestinians of the 21st century. I remain skeptical, but open to good arguments why we would want those 200 miles of Mexican territory (since as far as I'm concerned it's all ours lent to them on sufferance after the Mexican War).