I had been reading about Tojo's history and was surprised that he was not Prime minister at the start of the war (Looking at other European Authoritarians and how they saw things through from start to end).
I do not understand why there were so many different people that held the office around the time.
I am making the assumption that Japan's participation in WW2 started with the Marco Polo bridge incident and subsequent invasion of China rather than Pearl Habour.
I have only been reading from Wikipedia, and no books specifically. I am rather new to having a more detailed interest in History.
At least from the publicly available documents, the substantive decision making authority relating to military affairs was concentrated from Nov. 1937 onward in the 'Imperial General Headquarters-Government Liaison Conference' which was later renamed to 'Supreme Council for the Direction of the War' informally called the 'Big Six committee'.
The committee kept on increasing in scope until by 1944, coinciding with the renaming, all substantive decision making authority on all affairs was concentrated there. See: Wikipedia:SupremeWarCouncil
Notice how the Prime Minister was only 1 of 6 yet was meant to assume ultimate responsibility with the Emperor while tradition prevents the Emperor from ever speaking in favor of the PM specifically.
In short, the PM position became increasingly untenable as the war dragged on and increasingly undesirable among the high ranking folks in Tokyo due to the, correct, perception that there was limitless downside with almost no upside as everything substantive had to go through the Army or Navy factions anyways at the end of the day.
Hence a very high circulation of people.