Yes....the government in Mexico...which cause us so much trouble...would be darned near as hard to change as Iraq's has been...
If I lived on the border, my land, actually on the border, I would build a fence on my property, US GOVT., be damned.
And, if enough of the landowners did that...then the left over "federal land" which is where we run into trouble politically, should be manageable with guards, drones, etc.
I think....lol
However, we have seen how Chavez is ..and if someone like him does get power in Mexico, it could go from bad to worse, because he wouldn't hesitate, I don't think...to really start a border war.
You are not thinking big, those big ranches in south Texas have miles of frontage on the Rio Grand River. Think of the cost of building a fence that illegals couldn't climb over or dig under and then maintain it. It's not going to happen. It would be a project for the government if it is possible at all. Maybe drones with cameras and a fleet of helicopters to be available to go when illegals were spotted by the drone. I don't know, the problem is huge.