You can't have stable expansion unless you're willing to kill/subdue the current occupants of Mexico and colonize it with English speaking Yankee governors and ubermenchen, and loyal legions (not hired local mercanaries) to maintain the new piece.
Are you willing to pay the price?
The context of my statement was a guess as to Bush's motives for his policy. I doubt that he would agree with your assumption.
On the other hand, if we adopt a harsh, military approach your assumption will be correct and we will have to do just what you propose.