”If Mexicans successfully reconquered parts of the south western United States, they would turn it into another Mexico and have all their problems.”
"If"? "Would"? It's already happened in vast swaths of the Southwest. Check out a census map by race of greater Los Angeles, especially to the East and Southeast, which is like an extension of Mexico. And sure enough, there are criminal gangs, hapless schools, etc. But it's all OK, don't you know, because there's a taco truck on every corner. Because whites who created modern society would be otherwise incapable of following a taco recipe.
Eat tacos or anything from those rolling slop-wagons at your own risk. The filthiness of Mexicans, their disdain for sanitation and healthy practices are legendary. Also TOUR those vast swaths of the Southwest. Seeing with your own eyes how those "people helping make America great" trashed so much trying to make their new home resemble the old. Just look at any Mexican slum. I'm sure you can find one close to you. Where we now live, we barely have a handful of Mexicans or blacks. Three or four "homes" as you enter the town literally looking like the proverbial city dump. With disabled vehicles and equipment all over the place. Visiting Houston friends asked me "when did we do a U-turn and get back to parts of Houston" when they saw the trash.
If the native people don't defend the land, and especially if they don't even try to, invaders can freely take it.
You're saying we should have shot the invaders? I'd agree but for the fact that would make some of us criminals. When we first moved to Houston nearly 40 years ago, we met a old rancher not far from the Rio Grande. We got to be friendly and were shown a secluded piece of ground. Where he had helped his father and grandfather bury illegals caught on their land.