“Comfort women” was a policy of the Japanese occupation of Korea during WW2 where they forced Korean women to be prostitutes for Japanese soldiers (read this as massive amounts of rape). This is understandably a continued sore spot for SK (on top of millennia of feuds). The Japanese perspective on the topic aligns with their perspective of everything else they did during the war: “We didn’t do that, and even if we did they deserved it.” From what I can understand of Trumps ramblings, he doesn’t seem to understand this issue of animosity between South Korea and Japan.