I recall that as well as the US, and the ROK (Republic of [South] Korea), my own country, Great Britain, and Canada, Turkey, Australia, Philippines, New Zealand, Ethiopia, Greece, Thailand, France, Colombia, Belgium, South Africa, The Netherlands, and Luxembourg took part. In the late 1960s I had a girlfriend whose (British) father was killed in Korea.
I quote from a BBC web site last updated in November 2009
"While atrocities conducted both by North and South Korean forces have already been documented, recently a much darker side to the US involvement in the Korean War has begun to emerge. It casts a shadow over the conduct of US forces during the conflict, particularly of officers and generals in command. Declassified military documents recently found in the US National Archives show clearly how US commanders repeatedly, and without ambiguity, ordered forces under their control to target and kill Korean refugees caught on the battlefield. More disturbing still have been the published testimonies of Korean survivors who recall such killings, and the frank accounts of those American veterans brave enough to admit involvement.
Kill 'em All': The American Military in Korea
By Jeremy Williams"
http://www.bbc.co.uk/history/worldwars/coldwar/korea_usa_01.shtml
I have to say that much of this is not exactly new - I remember reading, over 30 years ago, an account by a journalist of his experiences in Korea during the war. He visited a US battalion whose personnel was drawn from the Mid West and was horrified to see that some of the soldiers were using Korean farmers for target practice. He talked to their platoon sergeant, a WW2 veteran, who apparently deplored this (but had made no effort to stop it) and who said "There is no more cold blooded killer than your average American farm boy."
Also we have been familiar for decades with what went on in Vietnam, and going back further, in the Pacific during WW2. I remember reading how US soldiers in Vietnam were told not to feel bad about killing VC because Asian people had a different view about death - they didn't value human life as highly as Westerners do. Rubbing out a "gook" was not to be compared with killing a white person. Hardly surprising that this spilled over. I don't believe My Lai was unique.
Of course you won't hear or read about this kind of stuff in the mainstream media in any of the countries I mentioned. Britain did some very nasty stuff in Kenya during the Mau Mau emergency in the 1950s. Check out Hola Camp.