Or that time that we parked an army on the Rio Grande because a bunch of people from the US decided to settle in the Mexican territory of Texas? That was a whole thing and the President of the US at the time, James K. Polk, ran on a platform of "Manifest Destiny" -- that the US should span "from sea to shining sea." There were a whole host of other countries with interest in that territory, not to mention the Indian tribes who would be displaced by that policy. The US has had a lot of dark periods in our history, and we shouldn't let those periods displace us from the moral certainty we derive from the Declaration of Independence and things like The Bill of Rights.