Oh let me tell you about the Jeju Massacre. https://en.wikipedia.org/wiki/Jeju_uprising

The Jeju uprising and its repression were notable for its extreme violence; between 14,000 and 30,000 people (10 percent of Jeju’s population) were killed, and 40,000 fled to Japan.

The US basically let this all happen under their colonial control over Korea at the time.