My reply would be to ask a question to answer their question....”Why did we work to restore your nation and give you back your independence; should we not have killed your emperor and left your nation a burning wreck?” (and then go silent and walk away).
Sure, that's the obvious answer, but the matter goes to the Japanese cultural need to save face. It's not part of their makeup to be able to say you are sorry or to accept someones concern either.
We were traveling in the Kansai with the parents of our exchange student who lived in Fukui. At a shrine, the father tripped and fell, and bloodied his elbow in the process. Our natural tendency is to render aid and express our concern. Not in the Japanese culture. Our daughter who had been a student in Japan for a year, interceded, telling us that by expressing concern and being sorry for what happened we were heightening his shame. In their culture, he had screwed up, and any acknowledgment would only serve to make him more aware of his “mistake!”
It's the same with WWII and their not being accepting of the truth, because in doing so it would mean that they as a people had “screwed up” and therefore turning the issue around allows them to save face.