Germany
Germany is credited with what has been termed "the holocaust". You can find plenty of documentation online for the atrocities committed. The US Holocaust Memorial Museum is one place where you can learn more about it. After WW2, the slogan "never again" was used to indicate that we as a human race would not allow such things to happen again. And yet, we did. In China and Russia more people were killed after WW2 than during. If we fail to look at the horror, and understand it, how can we prevent it from happening again? How can we prevent ourselves from being swept up by similar currents and participating in it?
I encourage everyone to watch documentaries. Obviously, each one is presented through the biases of the documentarian that made it, but with enough viewpoints watched, we can get a good sense of what actually happened. The longer documentaries seem to be better: