In order to explain the strain of anti-American sentiment in South Korea, a Korean friend of mine once told me that "America acted like Nazis" in Korea, including massacres, violent suppression of political dissent, etc. I'm wondering to what extent this is true.
There was the 1948 Jeju uprising which happened during the American Occupation following the Japanese surrender. Basically, there was a ton of unrest in the power vacuum following the Japanese withdrawal. There were a lot of communist sympathizers on Jeju, and the us backed interim government in Seoul decided to put a rather bloody end to some rioting/uprising there. Upper estimates putt he number dead at up to 60k.
There were about 6 Americans on the island at the time, and the US was doing its damdest to get out of Korea by 49. At worst, the US was complicit it the deaths, or caused them through negligence of not stationing more troops there. It was a complicated time, made worse by Chinese, Soviet, and US interference.
Here is an interesting article I found on the incident. http://www.newsweek.com/ghosts-cheju-160665
Edit: "under the loving care of the fatherly leader" has a great synopsis of the time leading up to the Korean War.
They massacred about 200 civilians at No Gun Ri then suppressed information of the event for 40 years