Joy Chen is sitting on a bench outside a new museum about the medical atrocities committed by Japan’s Unit 731 in Manchuria during World War II, trying to absorb what she learned inside: After the war, the United States covered up Japan’s biological warfare research on humans, allowing the perpetrators to escape punishment and to prosper.

That is detailed prominently in exhibition notes and an audio guide in the black marble building that lies like a split box here in Pingfang, on the edge of Harbin in northeast China: “Out of considerations of its national security, the U.S. decided not to prosecute the leader of Unit 731 and the criminals under him. They all escaped trial for war crimes.”

Led by Dr. Shiro Ishii, Unit 731 bred plague microbes, then deliberately infected thousands of men, women and children. It conducted vivisection and frostbite and air pressure experiments, transfused prisoners with horse blood and studied the effect of weapons on the body, among many things.