With only dozens of "comfort women" alive today, a prominent historian of their plight called for international efforts to preserve the historical documents for future generations.

"A lot of the documents and photos of 'comfort women' were ruined by fire or water as the Japanese military tried to destroy the evidence when they were defeated," said Su Zhiliang, a history professor at Shanghai Normal University, who has spent more than two decades researching the "comfort women" system and campaigning on behalf of the victims.

The Japanese military set up the first "comfort stations" in Shanghai in 1932. By the time of the full-scale invasion in 1937, the military brothels where women were forced into sexual slavery were all over China, from northernmost Heilongjiang to southernmost Hainan Island, said Su during a visit to San Francisco at the invitation of the Comfort Women Justice Coalition in that city.

The "comfort women" system was expanded to Southeast Asia, until the end of World War II in 1945. The Japanese military forced an estimated 400,000 girls and women into prostitution.

Half of them were from China, 140,000 to 160,000 from South Korea and the rest were from Japan and other Asian countries, according to Su.

"Comfort women" documentation was not meant to protest against Japan but to preserve the "shared memory of humankind", said Su, also director of China's Comfort Women History Museum at Shanghai Normal University.

"What the Japanese military did to 'comfort women' is an atrocity against human rights, especially against women's rights. ... Not only Japan, but the whole world should reflect upon it," he said.

In 2015, Su was prominently involved in the Chinese application to have the Nanjing Massacre and "comfort women" documents inscribed on the UNESCO Memory of the World Register, a compendium aimed at preserving documented heritage of universal value.

While the historical documents of the Nanjing Massacre were listed on the program, the UNESCO Committee rejected the application of "comfort women" because of intensive behind-the-scenes lobbying from Japan.

In June, the applications were submitted by an international joint committee representing eight countries and regions - the Chinese mainland, Taiwan, South Korea, the Philippines, Indonesia, East Timor, the Netherlands and Japan - as well as three war museums in the UK, the US and Australia.

Su showed some of the key evidence filed with the application - including the records of the "comfort stations", photos taken by the Japanese soldiers, and a membership roster of a Korean natives association in Nanjing - at a lecture attended by activists from different communities in the San Francisco Bay Area.

In autumn, the UNESCO committee will discuss the application and make a decision.

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