The Nanjing massacre
Found via Boys Wear Pants.
These atrocities are extensively documented in Iris Chang's book The Rape of Nanking: The Forgotten Holocaust of World War II, which I have and which contains some of the same photographs. The book estimates that the Japanese killed almost twenty million civilians in China as a whole, many of them under circumstances of unspeakable brutality. Unlike Germany, Japan has never really officially acknowledged the magnitude of its crimes during the war.
I recall that several years ago I mentioned to a Chinese woman that many people think the American use of atomic bombs against Japan during World War II was immoral, and asked her what she thought of this. She said that this was the most ridiculous thing she had ever heard; she also said that in her opinion I could search the whole of China and not find even one person who would agree that the US was wrong to drop atomic bombs on Hiroshima and Nagasaki. Anyone familiar with the historical record would fully understand her views.