The formal term for wartime sex workers in Japanese is juugun ianfu, or frontline comfort women. In everyday parlance, the noun ian alone refers to recreation, such as company-sponsored R&R or facilities provided by companies as an incentive or to show appreciation to employees.
During the war, ianfu were installed in "comfort stations" across Asia, operated either directly by or under the aegis of the Japanese military.
While prostitution around military encampments has existed throughout history, imperial Japan's institutionalization of military brothels was notable for its enormous scale and cold, efficient brutality. Digging into defense archives, Chuo University scholar Yoshiaki Yoshimi unearthed irrefutable evidence of how the imperial and military authorities procured women much like rations or rifles, often recruited with advertisements for factory workers or other conventional jobs. While experienced prostitutes from Japan were initially sought, overwhelming demand soon led to "sourcing" locally in occupied areas including Korea, China, Indonesia, and the Philippines. Testimony from the victims and former Japanese soldiers paints a grim picture of females tricked or coerced even as minors into a life of indescribable misery.
At the so-called comfort stations, recalled veteran Yasuji Kaneko, "there would be a long line of soldiers waiting their turn … Each man would finish in two or three minutes, and then it was 'Next!'"
The comfort women system was based on the belief that it would discourage soldiers from raping civilians (a faulty assumption, as it turned out), control venereal disease, and prevent the leak of military secrets (since the brothels were isolated and tightly controlled by the military).
Exact numbers are impossible to come by, but it's been estimated that as many as 200,000 women across the Japanese empire were conscripted into sex slavery. Yoshimi's revelations eventually forced Tokyo to issue an official apology to the comfort women in 1993.
Despite overwhelming evidence to the contrary, Japanese nationalists continue to portray the comfort women as willing prostitutes. The comfort women issue has become another political tool for revisionists like Prime Minister Shinzo Abe, who seek to do away with the pacifist clause of Japan's constitution and insist that Japan is done apologizing for its wartime behavior.
Mayor Hashimoto stepped beyond the usual nationalist rhetoric, however, when he blurted out at a news conference and posted on Twitter his belief that soldiers have "sexual energy" that must be relieved, and even urged a U.S. commander in Okinawa to have his men patronize local red light districts, a suggestion that was quickly rebuffed.