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Breaking the Cipher of the Napoleonic war briefing, unread for 217 years

Photo: Hacker News

What happened

A letter to General Marmont, listed as unsolved and known only from a mis-dated 1969 reproduction, read in full: redated to March 1809, placed two weeks before Austria invaded Bavaria, and checked against the war that followed.

Summary assembled by rule from the sources below

Why it's spreading

Sources