Mexico as a country has been around less than three centuries. Before being an independent country, it belonged to the Spaniards after they themselves wiped out most of the native population. To say that there is land in the U.S. that once belonged to Mexico is meaningless. All the land in the world once belonged to someone else, including the entire continents of North and South America.

Having said that, I don't have a problem with immigration. The legal immigration process exists primarily for two reasons. 1) to ensure that the people entering the country are not criminals (or worse), and 2) to ensure that those entering the country have the means to support themselves.

So if you walk across the border, are not otherwise a criminal, and don't expect to be supported by taxpayers, then I don't personally have a problem. However, that does not accurately describe what is happening.