United States aerial reconnaissance of the Soviet Union
Beginning in 1946, United States Army Air Forces conducted aerial reconnaissance flights along the borders of the Soviet Union and other Socialist Bloc states in order to determine the size, composition, and disposition of Soviet forces.
The necessity of peacetime overflights was reinforced after the escalation of the Cold War in the late 1940s and, in particular, after the Korea War started in 1950. US President Harry S. Truman authorized selected overflights of the Soviet Union in order to determine the status of its air forces. It was feared that the Soviets might launch a surprise aerial attack on the United States with long-range bombers.
In 1952 a modified B-47B bomber made the first deep-penetration U.S. overflight of Soviet territory to photograph Soviet bombers in Siberia. (Limited coastal overflights had been conducted by aircraft from the US Air Force and the US Navy several months earlier.) This mission established that the Soviets were not massing bombers in eastern Siberia.
Overflights of the Soviet Union with the newly designated RB-47Es continued through 1954, often at great risk, as they were routinely intercepted by Soviet MiGs. It became apparent that a new aircraft was needed that could operate at altitudes well above any Soviet air defenses.
In November 1954, President Dwight D. Eisenhower approved a secret program under the direction of the CIA to build and fly a special-purpose high-altitude reconnaissance aircraft named Project AQUATONE. Lockheed was chosen to build the reconnaissance plane and in August 1955 the first Lockheed U-2 was test-flown.
Other strategic reconnaissance missions continued as the U-2 tests were ongoing. In early 1956 Project GENETRIX involved using high-altitude photo-reconnaissance balloons that were intended to collect photographic intelligence as they drifted across the Soviet Union.
During Project HOMERUN (between March and May 1956) RB-47E reconnaissance aircraft flew almost daily flights over the North Pole to photograph and gather electronic intelligence over the entire northern section of the Soviet Union.
On May 6, 1956, six converted bombers, flying abreast, crossed the North Pole and penetrated Soviet airspace in broad daylight, as if on a nuclear bombing run. Any Soviet radar operator seeing the bombers would have no way of knowing that the mission was an act of espionage and not of war.
On 4 July 1956, the first U-2 flight over the Soviet Union took place. Soviet leader Nikita Khrushchev angrily protested this overflight and feared that “when they understand that we are defenseless against an aerial attack, it will push the Americans to begin the war earlier.”  This prompted the Soviet Union to develop new air defense systems.
Strategic overflight reconnaissance in peacetime became routine U.S. policy. Project OXCART advanced aerial overflight reconnaissance capabilities with the development of the Lockheed SR-71 Blackbird.
Following the 1960 U-2 incident, Eisenhower ordered an end to American reconnaissance flights into the USSR. This policy was upheld by President Kennedy, who in 25 January 1961 told a press conference, "I have ordered that the flights not be resumed, which is a continuation of the order given by President Eisenhower in May of last year". In 1964 CIA head John A. McCone emphasized to the Johnson administration the orders were not a pledge barring further flights, but simply a directive that the flights not be resumed, one which can be countermanded. As there was no strategic situation requiring photography, Johnson elected to continue the policy. This was further reinforced by the rise in satellite reconnaissance.
In spite of the formal end to reconnaissance efforts, Project Dark Gene, a CIA-Iranian program of intrusions into Soviet airspace to explore Soviet air defence systems, continued up to 1979. The US also remained involved in reconnaissance of mainland China into the 1970s using Taiwan U-2 overflights.
More than 40 US aircraft were downed by Soviet forces and about 200 US servicemen were killed. Their families were given misinformation by the US military about how they died.
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