Not to put too fine a point on it, but Aztecs were considerably more brutal than the Spanish. And besides that, we (kind of) paid for the Mexican Cession, which, not that anyone really cares, had been part of Spain for three hundred odd years, and only part of Mexico for about thirty years, and was thinly populated, mostly by people who were neither Spanish nor Mexican. And, as GUEST^*^ points out, there was the Wilmot Proviso. So only Texas had slaves. And, say what you want, we helped the Mexicans get rid of the French(though poor Emperor Maximillian was as enlightened and progressive as Benito Juarez, and didn't get the fact that he was a a puppet of Napoleon III, and not the liberator of the Mexican people).
I mention this only because MGM is active in this thread, and I thought he'd appreciate mention of Maximillian because his wife was the real "Mad Carlotta", which I may have pointed out before...but we are all getting older, aren't we?