Secret Intelligence Service
A British national-level civilian organization, the Secret Intelligence Service (SIS), often but incorrectly called "MI6", is the British center for intelligence collection, covert action, and intelligence analysis. Its functions are roughly comparable to the U.S. Central Intelligence Agency, and, as the CIA is part of the United States intelligence community under the Director of National Intelligence, SIS coordinates its work through the cabinet-level Joint Intelligence Committee.
SIS can trace its ancestry back to operations by Sir Francis Walsingham, an advisor to Queen Elizabeth I. During the Second World War, it controlled the British signals intelligence organization, then the Government Code and Cipher School (now the Government Communications Headquarters (GCHQ)), which ran the vital ULTRA cryptanalytic program against German communications. Also during WWII, it competed, with an ad hoc covert operations organization, the Special Operations Executive (SOE), dissolved at the end of the war with some functions reverting to SIS. The proper balance between clandestine human-source intelligence and covert action remains a constant challenge for national policymakers.
Much as the CIA can call for assistance from the United States Special Operations Command, SIS has a working relationship with UK Special Forces (UKSF). In turn, SIS and CIA cooperate in many ways, such as jointly operating one of the best sources on the Soviet Union, Oleg Penkovsky.