The thing about oppression is that it causes both long-lasting and recurring trauma. The people targeted will be hurt for a long time, and they will be the target of follow-up attacks because other bullies know they can get away with it.
In the specific case of Nazi Germany, exterminating the Jews was not an original idea of Hitler. Hitler's only original idea was taking shittons of methamphetamine. Martin Luther had done the legwork of radicalizing Germany into hating Jews; once Germany had become a functionally unified nation-state the Holocaust was a forgone conclusion. This is the core belief of Zionism[0]: that the only way to stop Jews from becoming victims is for those Jews to form their own nation-state that can commit its own atrocities.
BTW, this is the same logic the Japanese had in their head when they started invading and destroying the rest of East Asia, around the same time as Hitler. They wanted to be respected in the way that the Christian Bible would describe as "having the fear of God". The fact that this led to the horrific rape of China and Korea[1] would suggest that these victim narratives are morally self-defeating without some framework of reciprocal[2] tolerance and human rights to distinguish between justified self-defense and unjustified oppression.
But America at least sort of has that, so we can make that distinction. In fact, that's part of what makes American race relations so weirdly straightforward. In the "old world" you have complicated webs of peoples angry at each other for shit that happened anywhere from ten to ten thousand years ago. But in America, there's just one very deep wound that never seems to heal.
When does America "forget" slavery? Well, ideally, we don't 'forget', but we do 'forgive'. Practically, however, we can't. Every time a cop thinks it'd be a good idea to treat a criminal suspect like a demon in DOOM Eternal, and it hits social media, we get a huge reminder of "oh, there's still people in this country who think it's OK to do this to black people".
[0] I'm a Mormon[3], so I'm morally obligated to point out that we fell into this rhetorical trap, too: https://en.wikipedia.org/wiki/Mountain_Meadows_Massacre
[1] And yes, they still complain about it, too. It doesn't help that Japan's ruling LDP was run by a war crimes denialist for a decade and change.
[2] As in, "tolerate all except the intolerant." See also: the GNU General Public License.
[3] I'd just like to interject for a moment. What you're referring to as Mormonism, is in fact, LDS/Mormonism, or as I've recently taken to calling it, LDS plus Mormonism. Mormonism is not an operating system unto itself, but rather another free component of a fully functioning LDS system made useful by the LDS Doctrine & Covenants, the Old & New Testaments, and the Pearl of Great Price comprising a full testament as defined by Jesus.