[Klaus]

I don't show you this to say "Germans were not that bad!" "The allies were bastards too..blah blah blah." We have all been through this discussion. Just have a look at it and let us know your thoughts...

Incidents that occurred during the involvement of the relevant nation in World War II include the following. Not all of these are agreed to be war crimes:

Canada
According to Mitcham and von Stauffenberg in the book "The Battle of Sicily", the Loyal Edmonton Regiment allegedly killed captured German prisoners during the fighting in Leonforte in July 1943. The incident is not mentioned in any other histories of the campaign however.
The Argyll and Sutherland Highlanders of Canada randomly burned houses in Friesoythe, northwestern Germany in April 1945 as a reprisal for the death of their commanding officer. The official historian of the Canadian Army, C.P. Stacey, noted in his autobiography that it was the only incident he was aware of that could be considered a "war crime" associated with Canadian soldiers in the Second World War.

United Kingdom
Area bombing of German cities, e.g. the bombing of Dresden and of Königsberg (East Prussia). While some historians consider this to be a war crime, other historians note that there were little protection for civilians in the laws of war at the time, or dispute that civilians were deliberately targeted, or consider this to be a justified means of waging war.

United States
Canicatti slaughter
Biscari massacre
Dachau massacre
Atomic bombings of Hiroshima and Nagasaki, Bombing of Tokyo, Bombing of Dresden, and other bombings. See Area bombing above. Regarding the atomic bombings, for example, one argument is that the alternative allied invasion of Japan, Operation Downfall, would have caused far greater loss of human life.

Free France
The "Marocchinate" of Cassino

Yugoslav Communist Partisan Forces
Bleiburg massacre
Foibe massacres

Soviet Union
Mass rape and other war crimes by Soviet troops during occupation of East Prussia, which also took place in parts of Pomerania (Danzig) and Silesia, and during the Battle of Berlin and the Battle of Budapest.
the Soviet Union had not signed the Geneva Convention (1929) relative to the Treatment of Prisoners of War. This may make it doubtful that the Soviet treatment of German and allied POWs, who "were [not] treated even remotely in accordance with the Geneva Convention", causing the deaths of hundreds of thousands, was a war crime. However, The Nuremberg Tribunal rejected this as a general argument, and held that the 1929 Geneva Convention was binding because it articulated general principles of international law that are binding on all nations in a conflict, despite one party's non-ratification of the Convention.
A violation of the laws of war that came to trial in Nuremberg and ended in a guilty verdict for Admiral Karl Dönitz but without a sentence for this crime, because the court heard evidence that both sides engaged in it:
Unrestricted submarine warfare incidents that occurred before the relevant nation became part of the allies:

Soviet Union:
Katyn massacre

Post World War II incidents involving Prisoners of War include:

United States:
Salina, Utah POW massacre
Rheinwiesenlager (disputed)