At the risk of paraphrasing He Who Must Not Be Named Online, I wouldn't expect to read a history book that said, "and so, after pitilessly slaughtering the badly outgunned Mexican Army, America had Santa Annas hands shackled in FRONT of him so he could sign the 'purchase agreement' more easily. " Certainly it would be odd for a TEXAS history book to record it that way (and we all know how important TX is in determining what the nations history books say. ) It may interest you to know that the principal Mexican objection to the Rio Grande as boundary with TX wasn't disagreement about what the peace treaty actually SAID, but the fact that Santa Anna was a bound prisoner when he signed his agreement (a fact he also raised himself to explain why he was President of Mexico AGAIN just ten years later, despite a peace treaty term that required him to forever renounce power. ) So, yeah, we cloaked our territorial gains in legal legitimacy, but, let's be honest here, after the capture of Monterrey Mexico had few options apart from full capitulation on whatever terms we deigned to offer. That we came back in another decade to "buy" more land only underscores that.

But that really goes back to the core of the question asked: It becomes permissible when you can get away with it, but whether it ever becomes truly moral may depend on whom you ask, and when. Which, if either, of those definitions fits what Dark Knight meant by "OK" only he and God can say. As a fellow Texan, I encounter this kind of sentiment regularly, and have found the best antidote to be rts: We don't take it because we don't WANT it; it wouldn't make anything better and would likely make things a lot worse (although it could get VERY interesting to see what happened globally when the US physically owned all of Mexicos substantial oil rights, but since we pioneered the practice of leasing it all to private entities pursuing a profit even if it means charging US taxpayers $4/gallon, I doubt much would change strategically. ) It's not exactly a dispassionate area to discuss, given the history (and current reality; most Mexicans will tell you they didn't HAVE a drug problem until wealthy US customers started buying contraband illegal in their own country; they certainly didn't lose national political leaders over it. )