Lee Ok-seon was running an errand for her parents when it happened: a group of uniformed men burst out of a car, attacked her and dragged her into the vehicle. As they drove away, she had no idea that she would never see her parents again.

She was 14 years old.

That fateful afternoon, Lee’s life in Busan, a town in what is now South Korea, ended for good. The teenager was taken to a so-called “comfort station”—a brothel that serviced Japanese soldiers—in Japanese-occupied China. There, she became one of the tens of thousands of “comfort women” subjected to forced prostitution by the imperial Japanese army between 1932 and 1945.

It’s been nearly a century since the first women were forced into sexual slavery for Imperial Japan, but the details of their servitude remain painful and politically divisive in Japan and the countries it once occupied. Records of the women’s subjugation is scant; there are very few survivors and an estimated 90 percent of “comfort women” did not survive the war.

Though military brothels existed in the Japanese military since 1932, they expanded widely after one of the most infamous incidents in imperial Japan’s attempt to take over the Republic of China and a broad swath of Asia: the Rape of Nanking. On December 13, 1937, Japanese troops began a six-week-long massacre that essentially destroyed the Chinese city of Nanking. Along the way, Japanese troops raped between 20,000 and 80,000 Chinese women.

The mass rapes horrified the world, and Emperor Hirohito was concerned with its impact on Japan’s image. As legal historian Carmen M. Agibay notes, he ordered the military to expand its so-called “comfort stations,” or military brothels, in an effort to prevent further atrocities, reduce sexually transmitted diseases and ensure a steady and isolated group of prostitutes to satisfy Japanese soldiers’ sexual appetites.

“Recruiting” women for the brothels amounted to kidnapping or coercing them. Women were rounded up on the streets of Japanese-occupied territories, convinced to travel to what they thought were nursing units or jobs, or purchased from their parents as indentured servants. These women came from all over Southeast Asia, but most were Korean or Chinese.

Once they were at the brothels, the women were forced to have sex with their captors under brutal, inhumane conditions. Though each woman’s experience was different, their testimonies share many similarities: repeated rapes that increased before battles, agonizing physical pain, pregnancies, sexually transmitted diseases and bleak conditions.

“It was not a place for humans,” Lee told Deutsche Welle in 2013. Like other women, she was threatened and beaten by her captors. “There was no rest, ”recalled Maria Rosa Henson, a Filipina woman who was forced into prostitution in 1943. “They had sex with me every minute.”

The end of World War II did not end military brothels in Japan. In 2007, Associated Press reporters discovered that the United States authorities allowed “comfort stations” to operate well past the end of the war and that tens of thousands of women in the brothels were forced to have sex with American men until Douglas MacArthur shut the system down in 1946.

By then, between 20,000 and 410,000 women had been enslaved in at least 125 brothels. In 1993, the UN’s Global Tribunal on Violations of Women’s Human Rights estimated that at the end of World War II, 90 percent of the “comfort women” had died.

After the end of World War II, however, documents on the system were destroyed by Japanese officials, so the numbers are based on estimates by historians that rely on a variety of extant documents. As Japan rebuilt after World War II, the story of its enslavement of women was downplayed as a distasteful remnant of a past people would rather forget.

Meanwhile, women who had been forced into sexual slavery became societal outcasts. Many died of sexually transmitted infections or complications from their violent treatment at the hands of Japanese soldiers; others committed suicide.

For decades, the history of the “comfort women” went undocumented and unnoticed. When the issue was discussed in Japan, it was denied by officials who insisted that “comfort stations” had never existed.

Then, in the 1980s, some women began to share their stories. In 1987, after the Republic of South Korea became a liberal democracy, women started discussing their ordeals publicly. In 1990, the issue flared into an international dispute when South Korea criticized a Japanese official’s denial of the events.

In the years that followed, more and more women came forward to give testimony. In 1993, Japan’s government finally acknowledged the atrocities. Since then, however, the issue has remained divisive. The Japanese government finally announced it would give reparations to surviving Korean “comfort women” in 2015, but after a review, South Korea asked for a stronger apology. Japan recently condemned that request—a reminder that the issue remains a matter of present foreign relations in past history.

Meanwhile, a few dozen women forced into sexual slavery by Japan are still alive. One of them is Yong Soo Lee, a 90-year-old survivor who has been vocal about her desire to receive an apology from the Japanese government. “I never wanted to give comfort to those men,” she told the Washington Post in 2015. “I don’t want to hate or hold a grudge, but I can never forgive what happened to me.”