I was thinking about this earlier but lets just imagine that the US gave back Ca, Az, NM and TX to Mexico. The one thing they will need is water and SoCal and parts of Az. get most of their water from the Colorado river, now in Co. Denver owns most of that water, in fact if you live near the river and are on a well you cannot use your well water to water your horses, it is for household use only!
I would bet there would be a court battle over the water flowing past Utah and the US would win, the Eastern states are already in a drout and with more people moving there they would need this water. This would leave SoCal and Az. with very little water causing most of the people there to leave.
Water is the most valuable commodity there is next to oil because you can't live without it!
I'm certainly no expert on water rights but it's an interesting thought.