Paradoxically, however, one interpretation of the missile crisis held that the decisive factor in its resolution had been America's nuclear superiority—that if the American nuclear arsenal had not been more powerful than the Soviet arsenal, the crisis might have turned out differently. Both sides subscribed to this interpretation at least in part, which led to a new round in the arms race just as both sides were moving closer to agreements on nuclear testing. During the mid-and late 1960s, the Soviet Union expanded its military expenditures so that by the end of the decade, Soviet Strategic Rocket Forces had a new generation of even more powerful ICBMs at their disposal. At the same time, the administration of President Lyndon B. Johnson abandoned the idea of seeking an overwhelming nuclear superiority and settled upon a new measure of nuclear striking power called "sufficiency." As defined by the administration it meant having the ability to survive a Soviet first strike with enough forces intact to retaliate with a devastating second strike. To do so, the emphasis would be placed on a better balanced triad structure of U.S. nuclear forces, consisting of missile, air, and naval strategic forces, together leading to the power to "assure destruction" to an adversary without engaging in a destabilizing arms race. Secretary of Defense Robert S. McNamara argued that such a structure was both cost effective and stable, and it was retained as the structure of the U.S. nuclear force until the end of the Cold War.
By the beginning of the 1970s, the nuclear forces of the Soviet Union and the United States were at relative parity. In terms of sheer explosive power the USSR had surpassed the United States and was in the process of developing weapons with even larger payloads and greater accuracy, but the United States retained the technological lead. With this parity came new challenges to deterrence theory. No longer did one side have a preponderance of strategic power, and it appeared doubtful that even a preemptive first strike would hold the advantage, since it was increasingly clear that neither side would survive a nuclear exchange without casualties measured in the millions. American policymakers quickly found, however, that the promise of mutual destruction in the bipolar contest with the Soviet Union was frustratingly ineffective in conflicts such as Vietnam, which fell outside of the strictly defined U.S.–USSR relationship.
Mutual assured destruction (MAD) lay at the heart of the Strategic Arms Limitation Talks (SALT) that began in Helsinki, Finland, in November 1969. The objective of the talks was not to reduce the arsenals of either side but rather to negotiate limits on future growth of those arsenals precisely to preserve mutual vulnerability. Two technological developments of the late 1960s threatened to destabilize the nuclear status quo: antiballistic missile (ABM) systems and the development of multiple, independently targetable, reentry vehicles (MIRVs) technology. ABM systems, as they were conceived at the time, were designed to protect cities from incoming missiles. Both the United States and the Soviet Union had developed first-generation ABM systems that could in theory, if not yet in practice, offer protection against first strikes. Partly to overcome such an advantage, both sides had invested considerable resources in developing the technology of MIRVs, a system whereby one missile could deliver several warheads to independent targets. Although these new technologies were designed to cancel each other out, in truth they threatened to destabilize the mutual destruction deterrent and spark off a new arms race, a race that would not only be dangerous, but expensive. The SALT process, therefore, was designed to limit these technologies and keep each side vulnerable to attack by the other.
With strategic nuclear war finally recognized as unwinnable, President Richard M. Nixon ordered Secretary of Defense James Schlesinger to review the military posture of the United States in light of recent technology. The result, known as the Schlesinger Doctrine, was essentially a refinement of flexible response, designed to balance Soviet bloc capabilities by threatening retaliation commensurate with the threat. Specifically, it enhanced the role of tactical nuclear weapons in a three-layered defense structure: conventional forces for conventional threats; tactical nuclear forces to counter tactical nuclear threats; and strategic nuclear forces to counter strategic threats. In essence, the Schlesinger Doctrine embraced what Henry Kissinger had proposed in the late 1950s: that a limited nuclear war was possible and was a desirable capability to have.
Despite President Jimmy Carter's efforts to further détente and continue the focus on nuclear sufficiency rather than superiority, the international and domestic political environments of the late 1970s actually pressured the administration to increase military spending drastically. During the presidential election campaign of 1980, the Republican candidate Ronald Reagan seized upon accusations made by prominent groups such as the Committee on the Present Danger, headed by Eugene Rostow and Paul Nitze, to accuse the Carter administration of allowing a window of vulnerability to open, claiming that détente had allowed the Soviets to gain a dangerous lead in the arms race to the point that even the hardened-silo Minuteman forces, the mainstay of the U.S. strategic missile force, were vulnerable to high-yield Soviet missiles. Reagan promised not only to neutralize that gap, but also to restore American military superiority and, to that end, deliberately strove to upset the balance of terror by focusing on defense rather than deterrence. The shift had important ramifications for the Cold War. Reagan reauthorized the development of the B-1 bomber and the next generation of highly accurate and MIRV-equipped Peacekeeper missiles to replace the aging Minuteman forces. He also authorized development of a controversial radiationenhanced weapon, the neutron bomb, which killed living matter but left nonliving matter relatively unscathed. At the same time, Reagan endorsed the recommendations of a high-level commission chaired by Brent Scowcroft calling for an evolution toward small, single-warhead ICBMs backed up by Peacekeeper missiles.
In 1983 President Reagan ordered a large-scale scientific and military project to examine the feasibility of a new generation of ABM defenses. Officially labeled the Strategic Defense Initiative (SDI), but more commonly known as Star Wars after the popular science-fiction movie, the objective was to develop a multilayered shield capable of stopping thousands of incoming ballistic missiles. In theory, lasers mounted on satellites, electromagnetic guns, and charged particle beam weapons would be used to shoot down incoming ballistic missiles anywhere from boost phase (soon after launch) to reentry (final descent to target). In championing the project, Edward Teller, the reputed "father of the H-bomb," made a dramatic and controversial return to the public debate of deterrence. Not only was the technology unproven, but it quickly became apparent that the price tag of such a system was almost impossible to predict and entirely impossible to pay. Not surprisingly, the Soviet Union reacted angrily to what seemed a blatant disavowal of the 1972 SALT Treaty. Nevertheless, Reagan ordered the project to proceed. For the remainder of the 1980s, the Reagan administration struggled to find a way to make SDI a reality while at the same time continuing to pursue meaningful arms reduction.