I read Barefoot Gen, the manga autobiography based on the life of Keiji Nakazawa and his experiences after the bombing of Hiroshima.
One scene that particularly resonated with me was Nakazawa's father telling his family that the Japanese government is foolish for entering this war and they will lose, to the surprise of his family who gasp in horror saying "No you can't say that! what if someone hears you?" The father goes on to say something along the lines of "The only way for Japan to prosper and modernize is to learn to work together with China and our neighbors, not by invading them."
Were many other Japanese citizens privately against the war and Japan's invasion of China as this illustration showed?
I think this is a difficult to answer question.
Firstly, in Barefoot Gen, his father is actually quite publicly against the war, and that's what got the Nakaoka family into trouble (like the jailing of the father or the other school kids destroying the Nakoaka's unfired pottery). So I do not think Daikichi Nakaoka is a good example of a Japanese citizen (real or otherwise) who was privately critical of the Japanese government, because he was also publicly critical.
And secondly, how would a historian really be able to figure out if "many other Japanese citizens" (emphasis my own) were privately against the war if, by the definition of the word privately, they kept those thoughts to themselves? The only source I think you would be able to trust would be diaries. But what constitutes as "many?" 1%? Because that would be 730,000 diaries or so you would need to find.