(Originally posted in February, but unanswered.)
I was reading through the Wikipedia article on the line of succession to the Japanese throne, and they mentioned in an uncited footnote that Prince Chichibu (second son of Emperor TaishÅ, brother of Emperor Hirohito, and a prominent militarist) was "Potentially ineligible to succeed by the terms of the 1889 Imperial House Law."
It's long since become a hypothetical question, but was this the case? Why? Or, is it at least a theory that appears elsewhere - and if so, where did it come from?
I was confused too when I saw your question, since Chichibu was very much an heir to the throne, but after reading the Wikipedia article in question, I've figured it out. The article's footnote misphrased what actually happened. He was completely eligible to succeed. But he was also eligible to be passed over after he was diagnosed with pulmonary tuberculosis in 1940.
The 1889 Imperial House Law regulation (Ch I, Article 9) that is relevant says that an otherwise-eligible heir suffering from "incurable diseases of mind or body" could be passed over in the succession, if the Imperial Family Council advised it and the Privy Council was consulted. According to Herbert Bix's biography of Hirohito, this was privately decided to be the case with Chichibu, although there was then and now much speculation this was also a very convenient reason to cut Chichibu out of the succession and importantly the potential regency for his nephew, the immediate heir. Chichibu was a lifelong sympathizer with some of the most extreme elements in the Japanese military, and his relationship with his brother the Emperor had been very rocky.