This story begins in the deep dark days of the Cold War,For the early Cold War period at least, "Berlin Operations Base" may be said to have been one of the most active and productive postings for CIA intelligence officers in Europe. Its first Chief of Base was Allen W. Dulles. Richard Helms succeeded Dulles in October 1945. Following in the shoes of these two future Directors of Central Intelligence were some of the most successful intelligence officers in the Agency--most of whom must remain anonymous even today. CIA Berlin was never an independent entity, however, but always was subordinate to the Senior Agency Representative in Germany.5 Moreover, the CIA mission in Berlin was never more than a very small part of the much larger Allied presence.
Across the city, in their compound in the Karlshorst district of Berlin, the Soviet intelligence services--in their various guises--moved in about the same time as their Western counterparts. Their mission always was dramatically different from that of the CIA and the Western intelligence services, however. Whereas for the Western Allies, Berlin was and would remain an important strategic intelligence base, the city provided no equivalent advantages for the Soviet services. The main foreign intelligence target for the Soviets was the US military presence in Western Europe, a target the Soviets shared with their East German counterpart in the Normanenstraße, the Hauptverwaltung Aufklärung (HVA) of the Ministerium für Staatssicherheit (MfS, or Stasi). Nevertheless, both sides used Berlin as an arena in which they could challenge the intelligence services of the opposing side. Moreover, the high level of intelligence activity in Berlin meant that counterintelligence problems always assumed a high priority, sometimes even overshadowing the more important "positive" mission of intelligence collection.6 It was partly because of Berlin's value as an intelligence base for America and its allies that the East German government eventually sealed off the western half of the city in 1961--a move that severely inhibited Allied intelligence operations there without incurring a similar disadvantage for the Eastern Bloc services.
What follows is a sampling of CIA intelligence documents dealing with Cold War Berlin from the beginning of the Allied occupation in the summer of 1945 until the construction of the Berlin Wall in 1961. This might be regarded as the classical period of the intelligence war in Berlin, when the relatively unrestricted access permitted between the eastern and western halves of the city facilitated the intelligence operations of both sides. It was during this period that Berlin earned its reputation as a "den of espionage," a reputation that at least partly lived up to the romantic image created over the years by novelists and screenwriters.With the partition of Berlin after World War II, the United States moved its military headquarters into the bombed-out city in July 1945. The American section was adjacent to the British zone of control in the area of Berlin known as Zehlendorf. From a sprawling residence there, the CIA, using the cover name BOB (Berlin Operating Base), began operations.
Berlin was also the main target of Soviet intelligence operations. The Russians reigned with an iron fist in their zone of influence, targeting the United States in particular and mounting a large-scale intelligence-gathering operation that included informers, double agents, and other people of interest in the city. The CIA ran two of its most important divisions at BOB—the secret intelligence branch and X-2 counterintelligence. If anyone was going to beat the Soviets at their own game in Berlin, it would be these two units.
As the BOB operations took root, they expanded into fields other than the usual acquiring of military intelligence. BOB agents concentrated for some time on the economic goings-on in the vast Soviet-controlled sectors of the city. Through the use of informers and other secret assets, BOB was able to penetrate the headquarters of Soviet intelligence in Berlin, which was located at Karlshorst. One of BOB’s most important successes was tracking the development of the Russian atomic bomb in the late 1940s. It reported to Washington that the Soviets were in the process of mining uranium ore, a vital component in the construction of a bomb.
https://www.cia.gov/library/center-for-the-study-of-intelligence/csi-publications/books-and-monographs/on-the-front-lines-of-the-cold-war-documents-on-the-intelligence-war-in-berlin-1946-to-1961/art-2.htmlhttp://warfarehistorynetwork.com/daily/military-history/operation-gold-the-cias-berlin-tunnel/
