In World War Two, surveillance of enemy communications provided essential material for BP’s Codebreakers and crucial intelligence for the Allies. Once messages had been decrypted and translated at Bletchley Park, the information they contained was meticulously recorded onto index cards. This vast index of data enabled intelligence to be generated on all levels of enemy activity – not just military plans and operations, but also information about individuals.
These cards are from an index of Japanese diplomatic messages compiled by the Japanese Military Intelligence Section in Block F. They contain records of senior Japanese and German personnel. Even personal details, such as an officer’s poor health or a family wedding, could provide valuable intelligence on where the enemy was and what they were planning.