Yes, the American government, and the American people, have committed atrocities in the past. Fortunately, I don't have to paint myself with that brush, since my ancestors didn't come to this country until the early 20th century.
But I challenge you to find ANY great nation which did not commit similar, or worse, atrocities in their past. Let's not forget the wrongs done to the peoples of Africa and Southeast Asia by the British, French, German and even Italian nations. Let's not forget the horrific actions of the Spanish Inquisition. Let's not minimize the malicious harm caused by the Crusades, the echoes of which are still reverberating today.
What one has to remember, though, is that those actions, while reprehensible to us now, were considered right and proper at the time. In most cases they were even sanctioned by the churches, whether Catholic or Protestant. In other words, they were not considered to be immoral acts then.
We consider them immoral now, though. Because we have grown as people and as nations. We have a better understanding of the importance of other people, regardless of race, creed or color. And, despite its flaws, we have America to thank for that.
And no, it's not perfect. There are still people out there who hate others simply because they look differently, or speak differently, or worship differently. Those kinds of people exist everywhere. But we can minimize their impact, I hope, by exposing their hypocrisy and hate for what it is. Barbarism.