What happened to MI1 - MI4?
MI5 was founded as the Home Section of the Secret Service Bureau. It was part of the War Office in the First World War, between 1916 and 1918. During this time it was renamed MI5, referring to its status as the fifth branch of the Directorate of Military Intelligence (see the FAQ "Where does the name "MI5" come from and why is this name still used?"). In the Second World War, MI5 was independent of the War Office, though it worked closely with it.
There were a number of MI (Military Intelligence) sections within the War Office's Directorate of Military Intelligence during both the First and Second World Wars. There were eventually ten MI sections during the First World War and seventeen by the end of the Second World War. The number of MI sections and their precise functions varied considerably as the demands of the war effort changed.
Few had anything to do with secret intelligence. For instance, MI4 during the First World War was responsible for supplying military maps, while MI9 during the Second World War helped Allied troops to evade and escape from behind enemy lines.
All of these sections, with the exception of our colleagues in the SIS (MI6), were later discontinued or absorbed into other organizations.
Source: https://www.mi5.gov.uk/faq
MI2, the British Military Intelligence Section 2, was a department of the British Directorate of Military Intelligence, part of the War Office. It was originally set up to handle geographic information. MI2a handled the Americas (excluding Canada), Spain, Portugal, Italy, Liberia, Tangier, and the Balkans. MI2b handled the Ottoman Empire, Trans-Caucasus, Arabia, Sinai, Abyssinia, North Africa excluding French and Spanish possessions, Egypt, and the Sudan.
After the First World War, its role was changed to handle Russian and Scandinavian intelligence. These functions were absorbed into MI3 in 1941.
MI4 was a department of the British Directorate of Military Intelligence, Section 4, part of the War Office. It was responsible for aerial reconnaissance and interpretation. It developed into the JARIC intelligence agency. The present day successor agency to MI4 is the Defence Intelligence Fusion Centre.[citation needed]
https://en.wikipedia.org/wiki/MI7 - propaganda and laison with British news media
MI8, or Military Intelligence, Section 8 was a British Military Intelligence group responsible for signals intelligence and was created in 1914. It originally consisted of four sections: MI8(a), which dealt with wireless policy; MI8(b), based at the General Post Office, dealt with commercial and trade cables; MI8(c) dealt with the distribution of intelligence derived from censorship; and MI8(d), which liaised with the cable companies. During World War I MI8 officers were posted to the cable terminals at Poldhu Point and Mullion in Cornwall and Clifden in County Galway, continued until 1917 when the work was taken over by the Admiralty. In WW2, MI8 was responsible for the extensive War Office Y Group and briefly, for the Radio Security Service.
MI9, the British Directorate of Military Intelligence Section 9, was a department of the War Office between 1939 and 1945. During World War II it was tasked with supporting available European Resistance networks and making use of them to assist Allied airmen shot down over Europe in returning to Britain. MI-9 infiltrated agents, usually by parachute, into occupied Europe. These agents would link up with a Resistance cell and organize escape-and-evasion efforts in a particular area, usually after being notified by the Resistance of the presence of downed airmen. The agents brought false papers, money and maps to assist the downed airmen.
