There is not one, single nation, not one, that cannot say that their people haven't committed a war atrocity. Beziers, Malmady, "Killing Fields", Nanking, Kilkenny, Wounded Knee, "Bataan Death March", My Lai, Manchuria, the Gulags, Warsaw Ghetto, Oradour sur Glane -- the list is endless and is not limited to Western countries or any particular time.

If you want to see war crimes, look to the history of any country, not just the US or Japan.