Nobody would claim that America has never made mistakes, never failed to live up to its own rhetoric. Nothing in the universe is perfect. There isn't a nation anywhere on the planet whose record for slavery and slaughter isn't worse. What the British did to the Irish and the Scots would have had their leaders doing the hemp dance right along with the Nazis at Nuremberg. And even the Swiss thought it was acceptable to inflict unspeakable cruelties on Gypsies and their children.
The difference, for better or worse, is that America never seems to stop examining and reexamining its historical failures, while other countries do their damnedest to sweep theirs under the rug and forget them.