You think you expelled the Latinos and stole the land? That's simplistic perspective. If you go read the real Mexican history books (serious scholars and historians) you will realize that many people in those state were pro-leaving Mexico. Why? Well, Mexican 19th century history is defined by the conflict between Federalism and Centralism where everything was to be controlled from Mexico City. You can guess how people living on the outskirts of Mexico took to taxations and impositions of politicians from the center of the country. Sure, the US swooped on this, but you can bet the people in those territories were not particularly bothered, au contraire. Also you have to factor in that Mexicans have not felt particularly united as a nation until after our Revolutionary War in the 1900's

> Moreover the US actually had more or less open borders for a long time

Well, what's changed is that the US is rich and traveling is pretty easy.

Moving to the US doesn't make you rich though, it merely gives you a better opportunity to become so. That has never changed.