According to Wikipedia, sourced from
The Nuclear Tipping Point: Why States Reconsider Their Nuclear Choices, Prime Minister Eisaku Sato, after the first Chinese nuclear test, told President Johnson privately that if China had the bomb, Japan should as well and that while public sentiment was against it, the Japanese people, the youth in particular, "can be educated", shocking Johnson, who made Japanese accession to the NPT a priority. Furthermore, it was suggested that tactical nuclear weapons could be seen as defensive weapons and therefore not prohibited by Article 9 of the constitution, and successive Japanese governments apparently tried to keep the possibility of building nuclear weapon open.
Although Sato would later adopt the Three Non-Nuclear Principles, he privately stated his expectation that in the event of an American war with China, nuclear weapons would be used immediately (
http://www.hiroshimapeacemedia.jp/mediacenter/article.php?story=20081226170930777_en), and his son released in 2009 a document on a secret pact with President Nixon to allow nuclear weapons on Okinawa after it's return to Japan (
http://www.hiroshimapeacemedia.jp/mediacenter/article.php?story=20091224152025789_en). Japan developing the bomb in the late 1960s, either as the first step in or because of resurgent Japanese nationalism or militarism, could be a starting point towards a more assertive and militarily stronger Japan on the world stage.