Polk started an expantionist war with Mexico.
And didn't go far enough with it.
So, you're a fan of the North American Union, I take it?
To be fair, if we
had gone further with it, we'd only be going in with Canada. A couple of good excuses later and we're knocking on Columbia's doorstep by 1870. Other than British Honduras (Belize) the rest of Central America would (maybe?) have been happy to have consistent rule of law and would have prospered after the invention of Air Conditioning like the US South, SouthWest and Texas have. Everybody would have been better off in the long run and that border (Panama/Comumbia) would be a LOT easier to maintain (short and swampy) security on.