ImDanGrow
Korea was treated very poorly by the Japanese during the early 20th century - (1900 - 1948)
During WWII and even earlier they would kidnap women and children and force them to pleasure their soldiers as relief women. While Japan has made apologies for a lot of atrocities, I don't really believe that they have apologized for those actions.
You can find tons of articles and wiki's out there for the conditions and the treatment of the Korean populace during the Japanese occupation. There exists survivors till this day who remember the atrocities and have a generation of people who were directly affected by their actions.
Heck, just read up on the Japanese chemical warfare trials that were performed against the Chinese - read up on Unit 731 and their experiments.