When Yamamoto talked about a 12 month war, it was because he knew that if a victory could be forced on the US and Commonwealth within a year of Pearl Harbor (ie we give up after Australia falls or any number of scenarios), then he could win. And after that year, we'd be on a full fledged war time footing, and the great big boots of the sleeping giant would be stopping across the Pacific. But to achieve that, it would have ment stripping much of the Japanese Army aviation out of China and just beating on Australia without mercy while inderdicting all shipping and building a lot of landing barges.

It also means he would have to not have political clowns second guessing him and telling him the rules he could use.