Ever heard of Manifest Destiny? It refers to the idea that it was America's destiny to spread across the entire continent of North America and to wipe away the Native American and Mexican populations. This idea was at its peak right after the Mexican-American War, which brought California, Utah, Nevada, Arizona, and New Mexico into the United States. And if there's any character in this book who thinks Manifest Destiny is the way to go, it's Captain White.
It seems tough for Captain White to talk about American Territory without getting choked up. As we read at one point:
We fought for it. Lost friends and brothers down there. And then by God if we didn't give it back. Back to a bunch of barbarians that even the most biased in their favor will admit have no least notion in God's earth of honor or justice or the meaning of republican government. (3.92)
He's angry that the U.S. hasn't done a good enough job of securing the territory it won in the Mexican American War. Plus he thinks the Mexicans are a bunch of barbarians who don't deserve to run their own country.
With all that said, it's tough to say exactly what Captain White thinks he'll accomplish with his band of forty-or-so men. Does he really think he's going to conquer all of Mexico because God has promised victory to the white man? Well it doesn't matter either way because White ends up getting his head chopped off and put into a jar. So the book doesn't exactly reward his way of thinking, unless getting your head pickled in a jar is your idea of a happy ending.