Here are some of the sources brought to my attention:
Wartime propaganda is not concerned with truth. It is concerned with painting the enemy in the worst possible terms. The victors write the histories, and after enduring a war victor populations don't want to read that their side's behavior was no different, and possibly worse, than the cruel and inhuman behavior of the vanquished.
Obviously, this puts limits on what historians can write if they want readers and university careers. It is only many years later that truth can begin to make its appearance.