MI1 or British Military Intelligence, Section 1 was a department of the British Directorate of Military Intelligence

Directorate of Military Intelligence

The Directorate of Military Intelligence was a department of the British War Office.Over its lifetime the Directorate underwent a number of organisational changes, absorbing and shedding sections over time.- History :...

, part of the War Office

War Office

The War Office was a department of the British Government, responsible for the administration of the British Army between the 17th century and 1964, when its functions were transferred to the Ministry of Defence...

. It was set up during World War I

World War I cryptography

Codes and ciphers were used extensively in World War I. The decoding by British Naval intelligence of the Zimmermann telegram helped bring the United States into the war....

. It contained "C&C", which was responsible for Code Breaking.

Its subsections in World War I were:

• MI1a: Distribution of reports, intelligence records.
• MI1b: Interception and cryptanalysis.
• MI1c: The Secret Service/SIS.
• MI1d: Communications security.
• MI1e: Wireless telegraphy.
• MI1f: Personnel and finance.
• MI1g: Security, deception and counter intelligence.

In 1919 MI1b and the Royal Navy's (NID25) "Room 40

Room 40

In the history of Cryptanalysis, Room 40 was the section in the Admiralty most identified with the British cryptoanalysis effort during the First World War.Room 40 was formed in October 1914, shortly after the start of the war...

" were closed down and merged into the inter-service Government Code and Cypher School (GC&CS), which subsequently developed into the Government Communications Headquarters

Government Communications Headquarters

The Government Communications Headquarters is a British intelligence agency responsible for providing signals intelligence and information assurance to the UK government and armed forces...

 (GCHQ) at Cheltenham.

Oliver Strachey

Oliver Strachey

Oliver Strachey , a British civil servant in the Foreign Office was a cryptographer from World War I to World War II....

 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

Room 40

In the history of Cryptanalysis, Room 40 was the section in the Admiralty most identified with the British cryptoanalysis effort during the First World War.Room 40 was formed in October 1914, shortly after the start of the war...

.