In 1942, the Nazis decided that forced laborers in concentration camps would work harder if they were promised sex - so they made female prisoners work in brothels for them.
The brothels form the subject of "Das KZ Bordell" (The Concentration Camp Brothel) by Robert Sommer, a book that has been hailed as the first comprehensive account of a little known chapter of Nazi oppression in World War Two.
Sommer's 460-page work, due to be presented at the Berlin state parliament on Wednesday, explores the origins, structure and impact of the "Sonderbauten" (special buildings) run by Heinrich Himmler's SS in Germany and Nazi-occupied Europe.
"In the collective memory and written history of World War Two, the camp brothels were for a long time taboo," the 35-year-old Berliner told Reuters. "The former prisoners didn't want to talk about it: it was a difficult subject to handle.
"It didn't fit so easily into the postwar image of the concentration camps as monuments to suffering."
Beginning with the Austrian camp at Mauthausen in 1942, the SS opened 10 brothels, the biggest of which was in Auschwitz, in modern Poland, where as many as 21 women prisoners once worked. The last opened in early 1945, the year the war ended.
The chapter is separate from the annals of the Holocaust of European Jews. Jewish women were not recruited as prostitutes, and Jewish men were not admitted to the brothels.
Sommer estimates around 200 women inmates in total were forced to work in the brothels - initially offered the prospect of escaping the brutality of the concentration camps.
"They were promised release after half a year if they served in the brothel. But the promises were never honored," he said. "Later, the SS just selected women they felt were suitable."
"Jews were not allowed in. Neither were Soviet prisoners of war," he added. "Jewish women did not serve as sex workers."
Tens of thousands of captured soldiers, political prisoners and people branded socially undesirable by the Nazis, including Roma and homosexuals, were held in camps alongside the millions of Jews who died in the Holocaust.
"The idea behind the brothels was to raise productivity by providing forced laborers with added incentive," said Sommer. "Yet from what I found, it didn't work at all. Only a few people were actually in a physical condition to go to them."
According to Sommer, the use of prisoners to provide sex to other prisoners was purely a Nazi phenomenon in the war.
The prostitutes, most in their early 20s, received more food and were treated less harshly than other women inmates.
In return, they had to provide sex to selected prisoners every evening between 8 and 10 P.M., and on Sunday afternoons.
"The brothels show another dimension to the Nazi terror, where victims of the Nazis were made into perpetrators against the women," said Sommer, who grew up in communist East Germany.
After the war, the women - many of whom had been interned by the Nazis on the grounds they were "asozial" or anti-social - remained stigmatized despite their ordeal, Sommer said.
The brothels were strictly regulated, charging a fixed sum. The idea of providing material incentives for prisoners was borrowed from Soviet gulags, where inmates' behavior could determine the size of their food rations.
"The Nazis even imposed race laws inside the brothels," said Sommer. "Germans who wanted to go to a brothel could only go to a German woman. And a Slavic prisoner only to a Slavic woman."
Only privileged prisoners like Kapos (camp supervisors) had the means to afford frequent visits, and Sommer estimates less than 1 percent of the camp population ever went to the brothels.
Once the SS had issued a brothel permit, men were assigned a woman and medically examined. If their name was read out in an evening roll call they were marched to the building and had a medicinal cream applied to their genitals by a doctor.
Even the act of intercourse was supervised, as detailed SS logs of the brothels testify.
"The SS had spy-holes to check up on them," said Sommer. "Only 15 minutes' sex and the missionary position were allowed."
To research the book, Sommer visited all 10 camps - which included Dachau and Buchenwald - and interviewed 30 former prisoners, among them a number of men who used the brothels.
However, nearly all the women forced to work there are now dead, and those that remain are reluctant to talk.
"We don't know of any who were compensated for what they went through," Sommer said. "It's important that these women are given back some of their dignity."
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