If the United States doesn't embrace and absorb Mexico, China will.
Bush is playing defensive geostrategic chess, making moves that are aimed at heading off negative consequences twenty years in the future and that are totally inexplicable to the amateurs watching the game - and still less to the idiots in the MSM. Absorbing Mexico is going to cause us a lot of short to medium term pain - walling Mexico off and abandoning it to preadatory villains in other parts of the globe will cost us the whole country later on. Lesser of two evils.
We don't have to like it, but we probably have to do it.