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I know that before Pearl Harbor, the British and Americans had broken the Japanese diplomatic code, but not the naval code (since before war started there weren't enough messages to analyze). However, just a few months after the war began, both the British and Americans were able to decipher the Japanese naval code JN-25. This allowed the British to anticipate the Indian Ocean Raid in April 1942 and for the Americans to anticipate Coral Sea and Midway in May and June. But how early were the Allies able to decode Japanese naval communications? March? February seems too early, since the Darwin raid was a surprise.
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