Atrocities that happened in the USA (if that made sense)?

Hey guys, I'm doing a project for my AP Lit & Compo cl (after the AP exam which makes no sense to me -____-), and we have to do a powerpoint presentation of "events that have occurred in the United States that can be considered atrocities that should never be repeated (i.e., Dawes Act, Chinese Exclusion Act, Relocation of Japanese...), or some event that took place to bring about the end of an injustice (i.e. The Seneca Convention...)". Can anyone give me a concise list of these types of events to choose from (especially some where I don't have to explain much about it)? I can't seem to think of any besides what's listed. :/ Please and thank you!