from strategy Mutual assured destruction In the event, technological developments supported the second strike. Initially, long-range bombers had to be kept on continual alert to prevent them from being eliminated in a surprise attack. When ICBMs moved into full production in the early 1960s with such systems as the U.S. Titan and Minuteman I and the Soviet SS-7 and SS-8, they were placed in hardened underground silos, so that an unlikely direct hit would be required to destroy them. Even less vulnerable were submarine-launched ballistic missiles (SLBMs) such as the U.S. Polaris and the Soviet SS-N-5 and SS-N-6, which could take full advantage of the ocean expanses to hide from enemy attack. Meanwhile, attempts to develop effective defenses against nuclear attack proved futile. The standards for antiaircraft defense in the nuclear age had to be much higher than for conventional air raids, since any penetration of the defensive screen would threaten the defender with catastrophe. Progress was made using surface-to-air missiles (SAMs), such as the U.S. Nike series, in developing defenses against bombers, but the move to ICBMs, with their minimal warning time before impact, appeared to render the defensive task hopeless. Then, during the 1960s, advances in radars and long-range SAMs promised a breakthrough in antiballistic missile defense, but by the early 1970s these in turn were countered by improvements in offensive missiles--notably multiple independently targeted reentry vehicles (MIRVs), which could swamp any defenses. (The first MIRVed ICBMs were the U.S. Minuteman III and the Soviet SS-17.) Measures of civil defense, which could offer little protection to the civilian populace against nuclear explosions and, at best, only some chance of avoiding exposure to nuclear fallout, also appeared hopeless in the face of the overwhelming destructive power being accumulated by both sides. By the mid-1960s fears had eased of a technological arms race that might encourage either side to unleash a surprise attack. For the foreseeable future each side could eliminate the other as a modern industrial state. Robert McNamara, the U.S. secretary of defense for much of that decade, argued that so long as the two superpowers had confidence in their capacity for mutual assured destruction--an ability to impose "unacceptable damage" defined as 25 percent of population and 50 percent of industry--the relationship between the two would be stable. The need to maintain strategic stability influenced the strategic arms limitation talks (SALT), which began in 1969 and became the centrepiece of President Richard M. Nixon's policy of détente with the Soviet Union. In 1972, with the ABM Treaty, the two sides agreed to ban nationwide antiballistic missile systems, thereby confirming the primacy of the offense. Attempts to consolidate the strategic standoff with a treaty limiting offensive weapons proved more difficult. (In 1972 only an interim freeze had been agreed.) The second round of talks was guided mainly by the concept of parity, by which a broad equality in destructive power would be confirmed. However, the difficulty in comparing the two nuclear arsenals, which differed in important respects, resulted in long and complex negotiations. A treaty called SALT II was agreed on in June 1979, but by this time détente was in decline, and it was dealt a final blow with the Soviet intervention in Afghanistan at the end of that year. In addition, the strategic underpinnings of arms control had been undermined by a growing dissatisfaction in the United States with the principles of mutual assured destruction. Copyright © 1994-2001 Encyclopædia Britannica, Inc.