MI1
MI1 or British Military Intelligence, Section 1 was a department of the British Directorate of Military Intelligence, part of the War Office. It was set up during World War I. It contained "C&C", which was responsible for code breaking.
Its subsections in World War I were:
In 1919 MI1b and the Royal Navy's "Room 40" were closed down and merged into the inter-service Government Code and Cypher School, which subsequently developed into the Government Communications Headquarters at Cheltenham.
Oliver Strachey was in MI1 during World War I. He transferred to GC&CS and served there during World War II. John Tiltman was seconded to MI1 shortly before it merged with Room 40.
