MI6 is famed as the place where James Bond and George Smiley worked, and most have heard of Britain’s MI5 which is the agency responsible for counter-intelligence. So what was MI2? That was the department tasked with spying on the United States.
MI stands for military intelligence and came about during World War I as to describe the departments of the Directorate of Military Intelligence which was part of the British War Office. There were a total of 21 of them. MI4 handled maps, for instance.
There was a reorganization in 1965 and only MI5 and MI6 remain as terms in use and only because of popular culture.