Francis Gary Powers's fateful flight, codenamed Grand Slam, was actually at the tail-end of American manned overflights of Soviet territory. The US and its allies had actually conducted a range of aerial reconnaissance missions against the USSR that started in the early 1950s. Most of these missions involved "tickling the Soviet bear," by flying to the edge of Soviet airspace and making a quick dash in and out of Soviet territory to gauge Soviet response times as well as photographing installations. Most of these flights involved converted and stripped-down bomber aircraft like the RB-47 and while they were quite hard for the Soviet PVO (the Soviet air defense network) to catch, the Soviets did shoot down a few. Lockheed's development of the U-2 was partly a response to the increasing effectiveness of Soviet air defenses and would allow for a more ambitious overflight of Soviet territory. The CIA operators in charge of the overflights certainly expected the Soviets to fire on the U-2s, but not shoot them down.

The early U-2 was in more than a few ways a high-altitude jet-powered glider. The massive wing allowed it to reach unprecedented high altitudes. These heights posed a problem for the early Cold War Soviet air defenses. For one thing, manned jet interceptors could not reach these altitudes easily, much less be in a position to fire missiles. The SA-1 SAM likewise had problems reaching this height to be effective. But the advantage in height also rendered some of the early generations of Soviet radar ineffective. The U-2s also had the advantage that not only did its performance envelope strain the Soviets' response times, its capabilities got better as the U-2 burned off its fuel. A U-2 overflight could reach altitudes of around 75000 feet. In contrast, the top operational ceilings for Soviet interceptors was under 60000 feet and their swept-back small wings provided little control in the thin air as their jet engines were at full-throttle.

Nonetheless, the Soviets did try to shoot down the initial U-2 overflights despite these handicaps. Although the early models of U-2 were not damaged, the pilots often likened these overflights in 1956 and 1957 to being in middle of a shooting gallery. The Skunk Works actually began upgrading the U-2 in 1958 with more powerful engines that were more responsive at altitude and began coating the U-2 with black "Iron Ball" paint that contained iron which would dissipate Soviet radar.

The Soviets did not slouch either. In addition to fielding the new SA-2 SAM system and upgraded early-warning radars that provided greater warning time of a U-2 overflights. Soviet interceptors also began training in snap-up attacks with their own high-altitude aircraft, the Yak-25 RV. Some of the U-2 pilots reported on the growing effectiveness and aggressiveness of Soviet air defenses, but the American intelligence community generally felt current upgrades were sufficient counters to the Soviets' efforts. One American National Intelligence Estimate (NIE) asserted that Soviet radars lacked effectiveness over 60000 feet and American intelligence in was unaware of the operational capability of the "Fan Song" radar that would guide the SA-2. The Air Force was more realistic about the SA-2 and its radars, but these estimates only reached the CIA a month before Grand Slam.

Despite the NIE assurances, the Eisenhower administration was still reluctant to authorize a resumption of Soviet overflights after briefly suspending them after the initial round of overflights. The American President was actually quite leery of overflights and the Soviet diplomatic protests that accompanied them, noting that the American public would see it as an act of war if the Soviets overflew American territory on the same scale. Nonetheless, both Sputnik and the NIE's estimates on Soviet ICBM capabilities prompted Eisenhower to authorize in 1959 a small number of U-2 overflights to accurately estimate the Soviets' actual missile capabilities. The NSC was also successful in getting Eisenhower to authorize more overflights than the initial single flight.

The resumed overflights were initially successful at evading interception. The newly-deployed SA-2s could still not quite reach the U-2s' altitude and the Soviet interceptors were likewise still handicapped. But the initial missions also provided the Soviet PVO operational experience with their upgraded weapons and radars. The PVO commanders also got more familiar with the likely routes and targets for the U-2s as well. The restricted number of overflights also meant that the U-2 missions would also have to fly over Soviet territory for much longer periods as their cameras had to pick up as many targets as possible. All of this proved to be quite useful in shooting down Powers. It should also be stressed that the PVO only brought down Powers only after an intensive effort to shoot him down. Oleg Penkovsky, a high-ranking GRU spy reported to the CIA that sixteen SA-2s were fired on Powers and one MiG-19 was the victim of friendly-fire. Powers's U-2 also had malfunctions in its autopilot and the May Day date of Grand Slam also meant not only was the PVO leadership concentrated in Moscow, but also meant there was little Soviet military air traffic to cloud the initial U-2 penetration.

The longevity of the U-2 airframe is a testament to the capabilities of the Skunksworks' product. But the U-2's extreme operational ceiling was a guard against interception, not a guarantee. The CIA planners in charge of the U-2 program did understand that they would have a brief window of opportunity before Soviet air defenses caught up with the U-2. Unfortunately for Powers, this window was much briefer than American intelligence anticipated. The Eisenhower administration understood that Soviet overflights were a calculated risk given their provocative nature and the capabilities of the PVO. In the case of Grand Slam, this was a calculated risk that did not pay off.

Sources

Burrows, William E. By Any Means Necessary: America's Secret Air War in the Cold War. New York: Farrar, Straus and Giroux, 2001.

Laurie, Clayton D. and R. Cargill Hall. Early Cold War Overflights, 1950-1956: Symposium Proceedings. Washington, DC: Office of the Historian, National Reconnaissance Office , 2003.

Peebles, Curtis. Shadow Flights: America's Secret Air War against the Soviet Union. Novato, Calif: Presido, 2002.

Polmar, Norman. Spyplane: The U-2 History Declassified. Osceola, Wis: MBI, 2000.

The short answer is that the US thought the U-2 flew too high for the Soviet defenses. The longer answer is only slightly more complicated. The US was very aware of the Soviet's SA-2 program, which in many ways was their Manhattan Project in terms of military importance and level of effort. This is because the US had a clear advantage in air power and aerospace technology, and the Soviet Union was actually very vulnerable to US strategic bombing for a very long time. The US was also aware of the SA-2 program and posessed estimates about the system's performance and capability.

The U-2 was designed based on the then-current philosophy of higher is safer. The plane's long wing made it the highest flying manned aircraft in the world, and there was not a single air defense platform that could reach it when it first flew. Radar could see it, and the Soviets frequently saw U-2's enter their airspace on their radar screens, but they were impotent in intercepting those overflights. This gave the US enough cover as the Russians couldn't produce irrefutable hard evidence of American aerial spying.

During the 1960 incident, Gary Powers' aircraft was engaged by 2 fighters and fired upon by no less than 3 SA-2's. Of that barrage only 1 hit the plane. The resulting political fallout prompted Eisenhower to cease the overflight program in order to promote his foreign policy agenda.

So was the U-2 a failure or a blunder by the US? No. US intelligence estimated that while they could be detected, Soviet air defenses could not shoot them down, which was repeatedly proven in the overflights prior to Powers. The CIA knew the risks and thought that those risks were worth the intelligence yielded. However, changing political circumstance lowered the acceptible threshold of loss to zero and the Soviets adapted their techniques to maximize their chances of shooting a U-2 down.

The US would continue its quest for altitude and speed in its aircraft design for a while after, developing the XB-70 Valkyrie and SR-71 as some examples. However, eventually the technique changed to low-level penetration to beat air defenses.