In 1972, the United States and the Soviet Union completed a series of discussions on nuclear arms limitations that resulted in a treaty limiting antiballistic missile (ABM) systems and an interim agreement to set maximum limits on intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). This SALT I agreement also led to a second round of talks on arms limitations. 
Gerald Ford and Leonid Brezhnev signing joint communiqué on the SALT treaty in Vladivostok, November 23, 1974
The tensions experienced during the Cuban Missile Crisis demonstrated to both nuclear superpowers the dangers of an unrestrained arms race. In the wake of that incident, U.S. President John F. Kennedy and Soviet Premier Nikita Khrushchev signed the Limited Test Ban Treaty, an important first step in what would become an ongoing effort to restrain the development and proliferation of nuclear weapons. Although that agreement limited atmospheric testing, it did nothing to constrain underground testing, which became more prevalent. It was followed by an Outer Space Treaty in 1967, in which the signatories agreed not to deploy nuclear weapons in space, and Nonproliferation Treaty in 1968, in which some nuclear nations pledged not to share nuclear weapons and non-nuclear states promised not to pursue the technology.
Even while these agreements were being negotiated, U.S. President Lyndon Johnson and Soviet Premier Alexsei Kosygin met in Glassboro, New Jersey to discuss setting up a more comprehensive series of negotiations to curtail the strategic arms race. At the time the talks began in 1969, both the United States and the Soviet Union had large second-strike forces, which meant if one country attacked the other, it could count on receiving an equally devastating counterattack. Equal second-strike capability was the essence of deterrence, but once it had been attained, there was no real reason that either country needed to continue to build additional weapons. Building ever-larger nuclear arsenals became a costly exercise.
The first two years of the talks between U.S. and Soviet negotiators focused on defining the types of technology that future arms control treaties would cover. Because both countries had worked on missiles and warheads independently, they had developed slightly different forms of the technology and any limits would have to apply to the equivalent systems in each country. Once this step had been completed, they began to discuss the actual limits that would be covered in the agreement. The doctrine of deterrence required that each side be able to maintain a large enough offensive force to destroy the other, but there was more room to negotiate limits to strategic defensive systems, which served to undermine deterrence by destroying second-strike capability. Both countries had developed anti-bomber defense systems that were far too established to be eliminated, but new antiballistic missile systems were just being implemented and provided an opportunity for compromise. The United States had developed first Nike and then Safeguard, both limited missile defense systems that could protect small areas, such as Washington, D.C., from ICBM attacks. The Soviets had similarly placed their own missile defense systems around the cities of Moscow and Leningrad. In 1972, the two countries agreed to limit the development of ABM systems to those already deployed, and this agreement took the form of a permanent ABM Treaty signed at the conclusion of SALT I. In 1974, the two sides agreed to limit the deployment of ABM systems even further to only one small covered area per country.
It was far more difficult for the two sides to reach agreements on practical limitations for offensive weapons beyond those required for second-strike capability. By the early 1970s, both the United States and the Soviet Union had developed far more nuclear warheads and delivery systems than either required to maintain deterrence. Although both had second-strike capability, the breakdown of their offensive forces was very different. Whereas the Soviet Union had more ICBMs and nuclear warheads stockpiled, U.S. missiles were more accurate. The United States began the decade with more SLBMs, although the Soviet Union quickly caught up and even surpassed U.S. levels. The United States remained ahead on long-range bombers, however, and the Soviet Union had nothing comparable to the U.S. B-52 bomber either deployed or in development. The United States retained one other advantage, and that was the fact that it had established "forward based systems" in Europe and East Asia, along with nuclear submarines, which together ensured that U.S. second strike capability was spread out across the world, not concentrated within U.S. borders.
The agreement the two sides reached set limits on the number of ICBMs, SLBMs, and missile-launching submarines each side could have. The limits were set slightly higher for the Soviet Union than they were for the United States to allow for both the building already underway and U.S. superiority in forward bases and long-range bombers. This situation was not ideal for either party, so they agreed to limit the timeframe of this part of the settlement to five years and in the meantime, to continue negotiations. Several members of the U.S. Senate were unhappy about the higher quotas granted to the Soviet Union, and they refused to ratify this interim agreement unless their approval included a statement demanding that all future agreements have "essential equivalence," or roughly equal quotas for both countries. With this point made, the Senate ratified the SALT I agreement.
The combination of the ABM treaty and the interim agreement on offensive strategic weapons did not mean an end to the nuclear arms race. The development of missile defense systems had led U.S. scientists to begin work on a new device that could mount multiple warheads on a single missile, and make it possible for each warhead to hit a different set target. These multiple independently-targeted reentry vehicles, or MIRVs, undermined the existing missile defense technology and ensured that, even without the ABM treaty, there would be few incentives to invest large sums in developing defenses. At the same time, however, the arrival of MIRVs on the nuclear scene made the concept of mutually assured destruction moot, as a country with a great many MIRVs would be tempted to strike first to destroy the opponent's missile silos and launch pads. Therefore this development struck at the heart of deterrence theory, making pre-emptive strikes not only possible but preferred.
To deal with this issue, in addition to the question of parity raised by the Senate ratification debate and concerns that continued technological advances would ultimately make the world less safe just as MIRVs did, the two countries embarked on a second round of Strategic Arms Limitations Talks, known as SALT II.
Gerald Ford and Leonid Brezhnev signing joint communiqué on the SALT treaty in Vladivostok, November 23, 1974 Even while these agreements were being negotiated, U.S. President Lyndon Johnson and Soviet Premier Alexsei Kosygin met in Glassboro, New Jersey to discuss setting up a more comprehensive series of negotiations to curtail the strategic arms race. At the time the talks began in 1969, both the United States and the Soviet Union had large second-strike forces, which meant if one country attacked the other, it could count on receiving an equally devastating counterattack. Equal second-strike capability was the essence of deterrence, but once it had been attained, there was no real reason that either country needed to continue to build additional weapons. Building ever-larger nuclear arsenals became a costly exercise.
The first two years of the talks between U.S. and Soviet negotiators focused on defining the types of technology that future arms control treaties would cover. Because both countries had worked on missiles and warheads independently, they had developed slightly different forms of the technology and any limits would have to apply to the equivalent systems in each country. Once this step had been completed, they began to discuss the actual limits that would be covered in the agreement. The doctrine of deterrence required that each side be able to maintain a large enough offensive force to destroy the other, but there was more room to negotiate limits to strategic defensive systems, which served to undermine deterrence by destroying second-strike capability. Both countries had developed anti-bomber defense systems that were far too established to be eliminated, but new antiballistic missile systems were just being implemented and provided an opportunity for compromise. The United States had developed first Nike and then Safeguard, both limited missile defense systems that could protect small areas, such as Washington, D.C., from ICBM attacks. The Soviets had similarly placed their own missile defense systems around the cities of Moscow and Leningrad. In 1972, the two countries agreed to limit the development of ABM systems to those already deployed, and this agreement took the form of a permanent ABM Treaty signed at the conclusion of SALT I. In 1974, the two sides agreed to limit the deployment of ABM systems even further to only one small covered area per country.
It was far more difficult for the two sides to reach agreements on practical limitations for offensive weapons beyond those required for second-strike capability. By the early 1970s, both the United States and the Soviet Union had developed far more nuclear warheads and delivery systems than either required to maintain deterrence. Although both had second-strike capability, the breakdown of their offensive forces was very different. Whereas the Soviet Union had more ICBMs and nuclear warheads stockpiled, U.S. missiles were more accurate. The United States began the decade with more SLBMs, although the Soviet Union quickly caught up and even surpassed U.S. levels. The United States remained ahead on long-range bombers, however, and the Soviet Union had nothing comparable to the U.S. B-52 bomber either deployed or in development. The United States retained one other advantage, and that was the fact that it had established "forward based systems" in Europe and East Asia, along with nuclear submarines, which together ensured that U.S. second strike capability was spread out across the world, not concentrated within U.S. borders.
The agreement the two sides reached set limits on the number of ICBMs, SLBMs, and missile-launching submarines each side could have. The limits were set slightly higher for the Soviet Union than they were for the United States to allow for both the building already underway and U.S. superiority in forward bases and long-range bombers. This situation was not ideal for either party, so they agreed to limit the timeframe of this part of the settlement to five years and in the meantime, to continue negotiations. Several members of the U.S. Senate were unhappy about the higher quotas granted to the Soviet Union, and they refused to ratify this interim agreement unless their approval included a statement demanding that all future agreements have "essential equivalence," or roughly equal quotas for both countries. With this point made, the Senate ratified the SALT I agreement.
The combination of the ABM treaty and the interim agreement on offensive strategic weapons did not mean an end to the nuclear arms race. The development of missile defense systems had led U.S. scientists to begin work on a new device that could mount multiple warheads on a single missile, and make it possible for each warhead to hit a different set target. These multiple independently-targeted reentry vehicles, or MIRVs, undermined the existing missile defense technology and ensured that, even without the ABM treaty, there would be few incentives to invest large sums in developing defenses. At the same time, however, the arrival of MIRVs on the nuclear scene made the concept of mutually assured destruction moot, as a country with a great many MIRVs would be tempted to strike first to destroy the opponent's missile silos and launch pads. Therefore this development struck at the heart of deterrence theory, making pre-emptive strikes not only possible but preferred.
To deal with this issue, in addition to the question of parity raised by the Senate ratification debate and concerns that continued technological advances would ultimately make the world less safe just as MIRVs did, the two countries embarked on a second round of Strategic Arms Limitations Talks, known as SALT II.
