Berlin has rightly been at the heart of every serious study of the Cold War. Germany's divided capital had strategic importance beyond any other foreign capital. Though military lines had ossified into a series of potential battlegrounds with border checkpoints, and a foreboding wall with watch towers sprouted where families once freely crossed, intelligence operations were much more fluid. As Richard Aldrich observed, ‘the Cold War was fought, above all, by the intelligence services.’ Intelligence officers from many countries found themselves in both East and West Berlin attempting to discern the next move of the North Atlantic Treaty Organization (NATO) and the Warsaw Pact. David E. Murphy, a former Chief of CIA's Berlin Base, and his unlikely co-author, former KGB Officer Sergei Kondrashev, characterised the city as a ‘battleground’ of enormous importance.

If the battleground was enormous, so were the audacity and scale of intelligence collection efforts within it. Between 1952 and 1956 the CIA and its partner, the British SIS, undertook the largest data exfiltration operation from a single point of presence in the pre-cyber era. The Berlin Tunnel, whose daring matched its logistical and technological complexity, was codenamed Operation Gold by the Americans and Stopwatch by the British. Its primary purpose was to discern indications and warning that would suggest the Soviets were planning a military invasion of Western Europe. The roots of the operation, however, were in the failure of another signals intelligence operation.

As world war turned to Cold War, both East and West refitted their intelligence machinery for the contest to come. In October 1948, the USSR clamped down on the poor information assurance practice of multiple uses of one-time pads to encrypt sensitive information before transmission thanks to the betrayal of William Weisband, a Soviet MGB (forerunner of the KGB) agent in the Russian section of the US Army's Security Agency (succeeding the Army Signals Intelligence Service) at Arlington Hall. In the context of the national emergency in the Soviet Union during its so-called Great Patriotic War, this vulnerability created by the one-time pads was perhaps understandable, but afterwards the Soviets readopted more systematic information security procedures to safeguard their most sensitive global intelligence operations.