It is one thing to say that in hindsight he held a wrong belief, and another thing entirely (especially without knowing his exact arguments and priors) to say that he held a wrong belief at the time and was not only wrong, but also therefore morally defective.

I would actually argue for the latter. Even if he might have had valid geopolitical reasons to advocate a nuclear strike (althought I highly doubt it), it would have been morally wrong nevertheless. In my opinion, there is never an ethical justification for the usage of atomic weapons. In fact, I firmly believe that the bombing of Hiroshima and Nagasaki was ethically wrong as well, but I know that other people have a different opinion on this matter.