Japan
konchan
I think most Americans that think about Japan's involvement in WW2 think about just a handful of things:
- Pearl Harbor
- Kamikaze pilots
- The first two atom bombs
I would guess that few people know about the atrocities they perpetrated, particularly in China and other surrounding nations, but also how they treated POWs; beheadings were common.
Unit 731 is particularly heinous. See the link below for details in human experimentation which was comparable or worse than any experiments Germany did.