IIDA, Nagano Prefecture—Hideo Shimizu has been frustrated by the reluctance of a peace memorial museum here to display panels about the infamous Unit 731, where he worked during World War II.

Shimizu found himself explaining to his wife more details about the building than what the museum offered. The building was the headquarters in China of the Imperial Japanese Army’s Unit 731.

Students' textbooks didn't mention the research at Unit 731 and other facilities until the 1990s — and even then, it was only after a legal battle that made it to Japan's Supreme Court. Now That's ...