MI6 (originally Military Intelligence 6) is the British Secret Intelligence Service. Often known by its initials, SIS is one of the British secret service agencies. SIS has a remit to conduct espionage activities overseas, as opposed to MI5 which is charged with internal security within the United Kingdom. It was founded (along with MI5) as part of the Secret Service Bureau in 1909. Its first director was Sir Mansfield Smith-Cumming, from whose initial the codename used by him and all subsequent directors of SIS, "C" (not "M" as in the James Bond stories), originated.
World War I
Its first significant test came with the First World War, during which it had mixed success. It was unable to penetrate Germany itself, but had some significant successes in military and commercial intelligence, achieved mostly with agent networks in neutral countries, occupied territories, and Russia.
Inter-War Period
After the war its resources were greatly reduced, and the organisation became MI6 in 1921. It began to operate mainly through a system of sometimes-grudging co-operation with the diplomatic service. Most embassies acquired a "Passport Control Officer" who was in fact the SIS head for that country. This gave SIS's operatives a degree of cover and diplomatic immunity, but the system probably lasted too long and was an open secret by the 1930s.
In the immediate post-war years and throughout most of the 1920s, SIS was preoccupied with Communism, and Communist Russia in particular. Sidney Reilly was loosely associated with SIS until his capture, and SIS sponsored and supported both his and Boris Savinkov's attempts to bring down the Communist regime.
Cumming died in 1923 and was replaced as "C" by Admiral Hugh 'Quex' Sinclair. Along with the rest of the intelligence community, SIS switched focus in the 1930s to Nazi Germany. Again its success was rather modest, though it did acquire several quite reliable sources within the German Government and Admiralty.
World War II
'Quex' Sinclair died in 1939 and was replaced as "C" by Lt. Col. Stewart Menzies. During the Second World War, SIS was overshadowed in intelligence terms by several other initiatives, including the massive cryptanalytic effort at Bletchley Park, the extensive "double-cross" system run by MI5, and the work of the Photographic Reconnaissance Unit.
Its most famous operation of the war was a spectacular failure known as the Venlo incident, in which SIS was thoroughly duped by agents of the German secret service, the Abwehr, posing as high-ranking Army officers involved in a plot to depose Hitler. Two SIS agents were abducted by the SS, which tarnished the service's reputation considerably.
Modern Era
Since 1997, SIS activities have been subject to scrutiny by Parliament's Intelligence and Security Committee. The SIS headquarters since 1995 is at Vauxhall Cross, located in Vauxhall in London, on the bank of the River Thames beside the Vauxhall Bridge.
The fictional spy James Bond supposedly worked for SIS (though the books by Ian Fleming should not be taken too seriously), and in fact the SIS building features in some of the Pierce Brosnan films.