I think what I need to really wrap my head around this is a better understanding of historical philosophies of land ownership. Presumably, at some point in human history, people wandered freely and if they found a place they wanted to settle they did...so long as nobody actively tried to stop them. And then at some later point in human history, people stopped doing that without thinking first, "but what if someone already owns this?" But when did that shift happen? Did it happen in Europe and then unhappen (for Europeans) in the New World? Why would it have unhappened?
I really wish I understood this better. From what best I can piece together, I've seen evidence for at least three different philosophies of land ownership:
1) Right of Discovery: If you saw it first, you own it. Of course, how do you define "saw" here. Countries were claiming huge swathes of land after barely setting foot in them...so this is kind of unsatisfactory.
2) Right of Improvement: Who cares if you've seen it, a man (or country) can only claim as much land as they can actually use, and you prove you're using it with things like irrigation, cleared forests, and generally making environmentalists cry.
3) Right of Defense: The Americans seem to have used this one against Spain in Florida...that no country can actually claim land they can't defend. Convenient, huh? The fact that I've invaded your country proves that I have a right to invade your country. Except...I can also see it turned around to make logical sense. In a warlike world, you don't want neighbors who can be easily invaded. See Russia & Poland, History of.
So, yeah. Obviously Europeans could have had no Right of Discovery in the New World unless you define away natives as non-people. Which it seems like they sometimes did. When it comes to the Mexican-American War though, it seems like you might have a clash between the Right of Discovery (Spain/Mexico) and the Rights of Improvement & Defense (America). But maybe I shouldn't say that without a more complete idea of what pre-cession Mexican culture in what is now north of the border was like. I feel like I have a pretty good grasp of California...but did you know Santa Fe was like, a city and stuff?
I guess I have always accepted that Mexico owned California and most of the west before the United States did, taking it was wrong, and that was that. It was really striking to me though that mission/rancho culture in California didn't exist nearly as long as I had always assumed, only 50 years or so. One of the things I took away from Battle Cry of Freedom was this fact: Texas only seceded after revolutionary Mexico tried to end slavery. Reading that, I filed it away in the folder I keep, "a million and one reasons why the United States never once had the moral high ground in history." After What Hath God Wrought though, I'm not so sure. I like revolutionary Mexico's emancipation policy, sure...but they also had some pro-dictator, anti-rule-of-law policies too that give me more empathy for the Americans in Texas that decided living under foreign rule wasn't so great anymore. In what sense did Mexico really "own" territory that was so sparsely populated? How do you even define Mexico...the government, however unstable it was? The collective sovereignty of the Mexican people? Did the people in Veracruz have a legitimate claim on land in Arizona that the Americans violated? Obviously, the United States has moral culpability for starting an offensive war, at the very least, but I'm trying to think of this in a more complex way than just reflexively feeling sorry for the trail of victims the United States has left in its wake.
This is an extremely messy post, I apologize. I can't really make a clear argument because I don't have one...just putting half formed thoughts out there for better scrutiny. With luck, maybe eventually I'll fill in all the gaps in my knowledge and make something of it.