I am stretching a bit with 200 years, and my point also brushes away a lot of tribal conflicts that came about from Manifest Destiny and the Civil War itself.

I am stretching a bit with 200 years, and my point also brushes away a lot of tribal conflicts that came about from Manifest Destiny and the Civil War itself. But the key point here is that some US leaders clearly understood it as a principle pretty early on even if they struggled to enact it at first.