That's that adage, people who don't learn from history are doomed to repeat it.
If one can rationalize crimes against humanity, then what's going to stop them from repeating it. They may decide to give some paybacks to US next time.
There's far, far more chance of Europe going on a massive Jew-killing spree than Japan invading Manchuria again. Being worried about the Japs when they are surrounded by so many nuclear powers is simply not realistic.