The comfort women system of the Japanese military began during the rape of Nanking and was repeated during World War II. were some 200,000 Korean and Chinese girls and women forced into sexual slavery by the Japanese government and used as prostitutes by the military. Hundreds of them were herded into caves and murdered with explosives.
"It is indisputable that these women were forced, deceived, coerced and abducted to provide sexual services to the Japanese military ... [Japan] violated customary norms of international law concerning war crimes, crimes against humanity, slavery and the trafficking in women and children ... Japan should take full responsibility now, and make suitable restitution to the victims and their families." -- International Commission of Jurists, November 1994 
Toru Hashimoto, mayor of Osaka, Japan, said:
To maintain discipline in the military, it must have been necessary at that time. For soldiers who risked their lives in circumstances where bullets are flying around like rain and wind, if you want them to get some rest, a comfort women system was necessary. That’s clear to anyone.