History of Japan–Korea relations - Wikipedia
https://en.wikipedia.org/wiki/History_of_Japan–Korea_relationsDuring the colonial period, more than 100,000 Koreans served in the Imperial Japanese Army. The service of these Korean men was forced upon them, ]. Approximately 200,000 Korean children (predominantly ages 12-17) were also sent forcefully as "comfort women" at the war frontlines to serve the Imperial Japanese Army as sex slaves. The issue regarding "comfort women" has been the source of diplomatic tensions between Japan and Korea since the 1980s.