And what were those western states before they were Mexican?<p>The US stole its original territory from England, who had stolen it from the previous inhabitants.  The US continued that trend--taking sovereignty over territory via application of force--until hitting the Mississippi.  Then, a rare purchase agreement between the US and Napoleon, who had taken land from Spain, who had taken it from rebels, who had taken it from Spain, who had taken it from a previous iteration of France, who had, of course, taken it from the previous inhabitants.<p>Southwest of Louisiana, that land was added to the US by frequent petitions to join the federation.  Previously independent, taken by rebellion from Mexico, who had previously taken it by rebellion from Spain and France, who had taken it from the previous inhabitants.<p>Mexico took this badly, and in the ensuing war with the US, Alta California changed sovereignty to the US, with monetary compensation to Mexico.  The land was previously held by rebels who objected to war with the US, who took it from Mexico, who took it from Spain, who, as usual, took it from the people who had already been living there.<p>So let&#x27;s not argue too much about who stole what from whom.  The elites like to redraw the maps, as the people on the actual land care little for the border invisibly whooshing over their heads.<p>And really, the US <i>did</i> pay Mexico $15M (in 1848-1853) for it, even though Mexico had <i>already</i> lost control of much of it to rebels and malcontents, <i>just like what happened in Texas</i>.  A bit later, the US purchased Mesilla from Mexico, and there is the modern border.<p>I doubt Americans would care much about Mexican immigration if our politicians had not selected immigrants as a scapegoat group.  They like to blame unemployment on worker competition instead of capital flight, because that makes it look less like their fault.