From the mid-1950s, criticism of massive retaliation became increasingly vocal. As Eisenhower well knew, the most challenging aspect of implementing massive retaliation was that it required a leap of faith on the part of the adversary that the United States would respond to localized and small-scale aggression by launching a nuclear strike, a reaction that was increasingly akin to suicide because of the rapid advances the Soviets were making in nuclear technology. As a consequence, there were a growing number of calls for the United States and NATO to bridge that leap of faith by modifying the strategy of massive retaliation to what retired British Rear Admiral Sir Anthony W. Buzzard called "graduated deter-rence." Only by being capable of responding in proportion to the threat, critics of massive retaliation argued, would nuclear threats become credible. Implicit here was a distinction between the tactical and strategic use of nuclear weapons, a distinction that massive retaliation explicitly disavowed. In 1957 Harvard professor Henry Kissinger elaborated on this argument by calling for increased investment in tactical nuclear weapons and acceptance of the possibility of limited nuclear war.
The observations of Buzzard and Kissinger were part of a trend toward public debate over nuclear policy. The increasing frequency of nuclear crises in the late 1950s and early 1960s and the growing absurdity of both superpowers' nuclear postures led to increased public concern with nuclear policy. For the first decade of the nuclear age, the American public had for the most part treated nuclear policy as "something best left to the experts," but by the end of the 1950s nuclear strategy had become a topic of public debate led by a cadre of increasingly visible professional strategists. Often civilians associated with think tanks such as the RAND Corporation, these professional strategists began to assume a new place in the U.S. military hierarchy and, in turn, in the public imagination. Scientists like Robert Oppenheimer, Edward Teller, and Werner von Braun had all become national figures through their contributions to the technology of the nuclear age, and by the late 1950s civilian professional strategists like Bernard Brodie, Henry Kissinger, Albert Wohlstetter, and Herman Kahn were becoming just as famous for their theorizing about how to use that technology. Although their fame most often came in the form of notoriety for their ability to discuss the absurdity of nuclear war in cold, calculating terms, they were nevertheless crucial for fueling the public debate. In the absence of hard evidence concerning Soviet decision making, these strategists were forced to form judgments about nuclear war without having any experience to draw on; thus, they substituted deductive hypotheses derived from the fields of political science, psychology, and economics for inductive historical experience. In a series of books, the most well-known of which is On Thermonuclear War (1960), Kahn challenged policy-makers and the general public to get beyond what he called "ostrichlike behavior" and to "think the unthinkable." His central point, as he put it in Thinking the Unthinkable (1962), was that "thermonuclear war may seem unthinkable, impossible, insane, hideous, or highly unlikely, but it is not impossible."
The presidential election of 1960 further propelled the public debate on deterrence. Since Buzzard's call for "graduated deterrence" in 1956, Eisenhower's political opponents had adopted the strategy under a revised name: flexible response. Maxwell Taylor, the army chief of staff in the Eisenhower administration, had for some time been a voice of dissent on massive retaliation and had expressed his concerns in his book The Uncertain Trumpet (1960), in which he called for a reprioritizing of U.S. defense spending to place more emphasis on the ability to control the escalation of crises. When John F. Kennedy was nominated as the Democratic Party's presidential nominee, he quickly adopted flexible response as the basis of his military program.
The Kennedy administration thus came to office basing much of its military program on a political refutation of the Eisenhower administration's strategy of massive retaliation. Despite campaign promises to institute ways to control escalation and thereby make crises "safer," the Kennedy administration quickly assumed an aura of being in perpetual emergency. From the failed invasion of Cuba in April 1961, the renewed Berlin crisis just months later, and the civil rights crisis at the University of Mississippi in the fall of 1962, it appeared to many that the administration was careening from crisis to crisis. The first practical test of flexible response came in the summer of 1961, when Soviet premier Nikita Khrushchev revived his ultimatum to end Western rights in West Berlin and thereby once again provided U.S. deterrence strategy with perhaps its most difficult challenge. With the crisis brewing, and concerned that he had undermined his own credibility through the Bay of Pigs imbroglio a few months earlier, Kennedy responded with a massive buildup of conventional forces in Europe in order, in his words, "to have a wider choice than humiliation or all-out nuclear action." At the same time, he reaffirmed NATO's nuclear guarantee to the city. In turn, Khrushchev quietly lifted his deadline, as he had two years earlier.
Of all the crises confronted during Kennedy's short presidency, the Cuban missile crisis proved the most dangerous, with the United States and the Soviet Union coming closer to the brink of nuclear war than ever before or since. When Khrushchev decided to deploy Soviet MRBMs, IRBMs, tactical nuclear weapons, and nuclear-capable medium-range bombers secretly in Cuba, where they would be positioned to strike most of the continental United States within minutes, his reasoning was to bolster the Soviet deterrent. Whether he wanted to use this deterrent in an offensive or defensive role has been debated by historians ever since. Once the deployments were discovered, Kennedy responded to the challenge by implementing a naval blockade of the island and threatening military action if the missiles and bombers were not removed. After a weeklong standoff, during which SAC's forces went on airborne alert, Khrushchev agreed to remove the missiles and a month later agreed to remove the bombers.
The crisis was resolved peacefully, but those who had witnessed the secret negotiations and the classified near misses had seen all too clearly how command and control might break down under crisis conditions. On the one hand, the resolution of the Cuban missile crisis without global destruction seemed to enhance the credibility of the deterrent on both sides. On the other hand, the missile crisis demonstrated that brinkmanship and ambiguity were simply too dangerous. Consequently, the crisis accelerated the momentum toward East-West détente. Formal negotiations to limit nuclear testing, which had been under way since 1958, finally bore fruit on 5 August 1963 in the form of the Limited Test Ban Treaty that effectively imposed mutual restraint on large-scale, above-ground nuclear weapons tests. And to reduce the risk of miscalculation and misinterpretation in a crisis, a communications hotline was established between the White House and the Kremlin.