ORIGINAL: el cid again
CID. Can you supply a source document for this statement? It seems rather odd that after months of planning and gaming Yamamoto would have suddenly had this "brainstorm" the day after the attack. And as the IJA wasn't offering 3 divisions for any Hawaiian advanture (look at what they were willing to make available for Midway with six months of victories behind them..., basically a regiment), the "landing" is just a pipedream.
This is not the way history is taught in the West - and you may imagine my surprise when I first read it. It was in a history of Hawaii called something wierd like "I Heard the Sea Call My Name". But it was a real history, and it cited the official Japanese history, an amazing (and almost impossibe to read) collection more than 100 volumes long which took decades to publish. Since then I have found a few more authors who have noted the basic story, including one actually naming the three divisions, in English. Probably the best brief way to become familiar with the massive materials without spending about a dozen years learning Japanese is to use University of Hawaii materials (they specialize in all Central Pacific history, as you might imagine), starting with The Pearl Harbor Papers and Hawaii Under the Rising Sun. These are scholarly - meaning you get good cites - and you then can follow up if you must in Japanese - or in archives (good luck there! we are not preserving the docs). [Commercial translation of Japanese costs $3000 a page, or $2000 if you order many pages. And it is almost never up to translating material of this age: no one understands the terms of art. The best path is to do it yourself and associate with scholars and linguists with an interest - that is everyone helps each other without charge - something you can only do if YOU bring something to the table. But such a process never ends - it only gets closer and closer to a sense of the material.]
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Well, I don't read Japanese, so that's a non-starter. But I do know that the IJA absolutely limited the number of it's units it was willing to commit to the Pacific Expansion, and I've never seen anything to imply that they were willing to commit 3 Divisions to Hawaii. But I'm sure the IJN did a number of case studies for attacks on Hawaii, Australia, New Caledonia, Fiji and all sorts of places---provided the Army would supply the troops. Which the Army virtually always refused to do. Might be the explanation for the "Plans" you mention above. I've seen "official" US plans to invade Mexico. But they are "official" only in the sense that the Army "WarPlans" Staff drew them up and periodically updated them for practice.
Given the confusion of materials and languages, it wouldn't be suprising that someone stumbled on some of these "Navy staff proposals" and thought he'd struck gold research-wise.