Activist Tamaki Matsuoka is challenging Japanese perceptions of the country's war record with a new documentary on the atrocities known as the Rape of Nanking.
Her film, Torn Memories of Nanjing, combines the memories of Japanese war veterans with accounts by Chinese survivors of the massacres of 1937-38, after Japan captured the former capital city of Nanking.
The film was shown at the Hong Kong International Film Festival on Sunday in its first screening outside Japan.
In the documentary, Matsuoka captures former soldiers admitting for the first time to mass rape and to the masscre of unarmed civilians in Nanking, which is now called Nanjing.
This runs counter to the accepted wisdom in her country, where history textbooks gloss over atrocities during Japan's invasion of China, and Second World War veterans are thought to have battled and lost an honourable fight .
"Chinese and Japanese perceptions of this war are totally different," Matsuoka said Tuesday. "That's why this documentary is called Torn Memories of Nanjing. My mission is to help more Japanese people learn the facts," Matsuoka said Tuesday.
Tomokazu Takeda, a young Japanese who helped produce the documentary, said he had no knowledge of the wartime experience of Nanking.
On Dec. 13, 1937, Japanese troops marched into Nanking and murdered 300,000 Chinese people, including many civilians, in a six-week orgy of violence. Chinese authorities say as many as 20,000 women were raped.
Japanese historians and journalists refer to the event as an "incident" and many deny the casualty rate was so high.
Takeda, who is in his 30s, said he was shocked by what he heard from Japanese veterans interviewed on film.
Juhei Teramoto, a former soldier, admits on film that "superior officers told us to commit robbery, murder, rape and arson and do anything."
"You know, we were young men and we were the ones who might die the next day, so we wanted to sleep with a girl."
Matsuoka is not a professional filmmaker, but her documentary, made with Takeda's help, is a powerful indictment of Japan's war record.
Former soldiers describe in detail how they worked as a team to hold down women and girls, checking their private parts for sexually transmitted diseases and drawing lots to decide who would go first.
Few veterans showed any remorse. Out of the 250 former soldiers interviewed, only three expressed regret for their actions, Matsuoka said.
The process had Takeda questioning his own family history. He remembered that his own grandfather was stationed in Nanjing after the killings.
"My parents told me that grandfather apparently was haunted by Chinese ghosts from time to time," Takeda said. "And he was often screaming, 'Chinese are going to attack me.'"
The documentary is the latest step in Matsuoka's 10-year campaign to expose Japan's atrocities in China.
She also has written newspaper articles, held photo exhibitions and brought victims to Japan to speak. Her long association with some of the individual veterans led them to agree to on-camera interviews.
It is a far-from-popular cause. She has been criticized by commentators and harassed by those who believe she is hurting Japan's image.
Chinese director Lu Chuan's 2009 feature film City of Life and Death and the 2007 documentary Nanking are among the films that have depicted the massacres.