Which is precisely why in that case the only solution is US Occupation of Mexico.
Not practical, Mexico is four times the size of Iraq and has almost four times the population. You would need to mobilize the entire Army, plus the entire National Guard and reserves as well as conscript hundreds of thousands of additional soldiers to properly occupy that country, plus protect the rest of our interests around the globe.
Plus it would literally be actual imperialism on a grand scale. I don’t see any stomach for that. Man the border and build an effective defense ( which may mean selectively occupy key terrain south of the border).
Something I recommend forty years ago. Either we change them or they surely will change us. Press 1 for English.