By way of Winston Churchill

The Grand Alliance, v.3 of his history of the Second World War, quoting the Nuremberg documents on the German invasion of Russia:

Hitler's main theme was that this was the decisive battle between the two ideologies and that this fact made it impossible to use in this war [with Russia] methods, as we soldiers knew them, which were considered to be the only correct ones under international law.

This event, of course, also generates Churchill's famous line, "If Hitler invaded hell I would make at least a favorable reference to the devil in the House of Commons." But for some reason this lesser-known quote seemed more worth reproducing.