Perhaps you've seen the signs and/or heard the rhetoric from immigration activists looking to legalize 12,000,000 illegal aliens (which by the way, is NOT a racist term). One of their latest slogans has become "We didn't cross the border, the border crossed us". This little catch phrase is so absurd it's a wonder that immigration activists haven't seen holes in their logic.
Unless you were born before 1848, when the US acquired the 4 border states after the Mexican-American War, I can assure you the border has not moved. And if you were born after 1848, then the chance that you were born in the USA is actually greater, since the border moved south. People crying that the border moved don't seem to understand the fact that if you were born in Mexico, the acquisition of Texas, California, New Mexico, and Arizona is completely irrelevant. Yes, Mexico would have a larger land mass, but Mexican land mass is not the issue. The issue is US citizenship, and those born south of the current border would lack US citizenship regardless of the national affiliation of the border states.
Let's assume for a moment that the US never acquired the border states. Does anybody believe that Texas, New Mexico, Arizona, and California would be as free and prosperous as it is today? Of course not - they'd have the standard of living equivalent to the Mexican states of Chihuahua and Sonora. Rather than crossing Rio Grande like they do today, Mexican immigrants would be crossing the Red River to get into Oklahoma.
In conclusion, the only people whose citizenship has been impacted by the 1848 land acquisition are US citizens who were born in a border states, and don't have the luxury of Mexican citizenship. Quite honestly, I suspect we could poll every resident of the 4 border states and use my right hand to count the people upset because they're US citizens rather than citizens of Mexico.