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There was basically no way to get Intel out of Japan. We had military attaches but they hid their important stuff from them.
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I recall reading what intel the US did get was mostly derived from the Chinese theater. Certainly the Flying Tigers air squadron supplied intel on the Zero's.
Problem was too often the intel weenies had their minds made up & nothing was going to change that. MacArthur's G2, Willoughby (sp?) was notorious for rejecting anything MacA felt went against his feelz. That was evident when the Chinese entered the Korea War.
US intel in the pre-WW2 era was solely lacking in competence. This continued into WW2, which is why Roosevelt invented the CIA with its Ivy League pedigree with Bill Donavan's crew.
Some might argue that military intel's issues continue to this day as far as rejecting things which go against their pre-conceived beliefs. I'll leave that to others to decide.