It's much easier to lay "righteous" claim to something when the majority of people laying claim look alike and reap the most benefits of that claim. No one except the ancestors of slaves wants to deal with slavery and what it means; how that legacy has affected people today. When you build a country on the backs of humans and then claim that you are the greatest country in the world that puts you in a slightly different cast than just another nation that has conquered a territory. 

The argument that the conquering of nations results in suffering seems reasonable if you're the victor. Had what’s been done to others, been done to the colonists, the U.S. would be in even greater debt, due to the reparations that would still be paid for European-American suffering.  Socialism would not be confused with communism, and universal healthcare would be the norm.

Sometimes you have to dig really deep to learn truth and even then truth can be considered unnecessary and burdensome if it shows the ruling class in a bad light. If we believed everything that we were taught in basic American history classes, we'd think that all Europeans are geniuses, the pilgrims were kind and that the Natives should have been more accommodating.