The Anti-Ballistic Missile Treaty, also known as the ABM Treaty or ABMT, was a 1972 arms control treaty between the United States and the Soviet Union on the limitation of the anti-ballistic missile (ABM) systems used in defending areas against strategic ballistic missiles, which are used to deliver nuclear weapons It was intended to reduce pressures to build more nuclear weapons to maintain deterrence. Signed in 1972, it was in force for the next 30 years. Citing purported risks of nuclear blackmail from a rogue state, the United States under the George W. Bush administration unilaterally withdrew from the treaty in June 2002, leading to its termination. In ICBM defense, the US has subsequently operated the Ground-Based Midcourse Defense ABM system based in Alaska and California, as well as the sea-based Aegis Ballistic Missile Defense System. Russia maintains the A-135 ABM system around Moscow, and has developed the S-500 missile system.
Under the terms of the original 1972 treaty, each party was limited to two ABM complexes, one for the nation's capital and one for an intercontinental ballistic missile silo field. Each ABM complex was limited to 100 anti-ballistic missiles and their launchers, two phased-array radars, and 18 smaller radars for early-warning. ABM missiles that were not static and ground-based were prohibited. In 1974, the limit was reduced to just a single ABM complex. The USSR chose to deploy the A-35 system around its capital Moscow, the US elected to deploy the Safeguard Complex around its ICBM fields of the Twentieth Air Force, although this was only operational for a year from 1974 to 1975. A 1997 treaty addendum permitted "theater missile defense": anti-ballistic missiles used against theatre ballistic missiles, as long as they were not tested against targets with velocities over 5 km/s, typical of ICBM terminal phase. Also in 1997, five years after the dissolution of the Soviet Union, its former member states Belarus, Kazakhstan, Russia, and Ukraine were established as successors to the USSR within the treaty, with one ABM system permitted between them.
Throughout the late 1950s and into the 1960s, the United States and the Soviet Union had been developing missile systems with the ability to shoot down incoming Intercontinental ballistic missile (ICBM) warheads. During this period, the US considered the defense of the US as part of reducing the overall damage inflicted in a full nuclear exchange. As part of this defense, Canada and the US established the North American Air Defense Command (now called North American Aerospace Defense Command).
By the early 1960s, US research on the Nike Zeus missile system had developed to the point where small improvements would allow it to be used as the basis of an operational ABM system. Work started on a short-range, high-speed counterpart known as Sprint to provide defense for the ABM sites themselves. By the mid-1960s, both systems showed enough promise to start development of base selection for a limited ABM system dubbed Sentinel. In 1967, the US announced that Sentinel itself would be scaled down to the smaller and less expensive Safeguard. Soviet doctrine called for development of its own ABM system and return to strategic parity with the US. This was achieved with the operational deployment of the A-35 ABM system and its successors, which remain operational to this day.
The development of multiple independently targetable reentry vehicle (MIRV) systems allowed a single ICBM to deliver as many as ten separate warheads at a time. An ABM defense system could be overwhelmed with the sheer number of warheads. Upgrading it to counter the additional warheads would be economically unfeasible: The defenders required one rocket per incoming warhead, whereas the attackers could place 10 warheads on a single missile at a reasonable cost. To further protect against ABM systems, the Soviet MIRV missiles were equipped with decoys; R-36M heavy missiles carried as many as 40. These decoys would appear as warheads to an ABM, effectively requiring engagement of five times as many targets and rendering defense even less effective.
The United States first proposed an anti-ballistic missile treaty at the 1967 Glassboro Summit Conference during discussions between U.S. Secretary of Defense Robert McNamara and Chairman of the Council of Ministers of the Soviet Union Alexei Kosygin. McNamara argued both that ballistic missile defense could provoke an arms race, and that it might provoke a first-strike against the nation fielding the defense. Kosygin rejected this reasoning. They were trying to minimize the number of nuclear missiles in the world. Following the proposal of the Sentinel and Safeguard decisions on American ABM systems, the Strategic Arms Limitation Talks began in November 1969 (SALT I). By 1972 an agreement had been reached to limit strategic defensive systems. Each country was allowed two sites at which it could base a defensive system, one for the capital and one for ICBM silos.
The treaty was signed during the 1972 Moscow Summit on 26 May by the President of the United States, Richard Nixon and the General Secretary of the Communist Party of the Soviet Union, Leonid Brezhnev; and ratified by the U.S. Senate on 3 August 1972.
The 1974 Protocol reduced the number of sites to one per party, largely because neither country had developed a second site. The sites were Moscow for the USSR and the North Dakota Safeguard Complex for the US, which was already under construction.
Missiles limited by the treaty
The Treaty limited only ABMs capable of defending against "strategic ballistic missiles", without attempting to define "strategic". It was understood that both ICBMs and SLBMs are obviously "strategic". Neither country intended to stop the development of counter-tactical ABMs. The topic became disputable as soon as most potent counter-tactical ABMs started to be capable of shooting down SLBMs (SLBMs naturally tend to be much slower than ICBMs), nevertheless both sides continued counter-tactical ABM development.
On 23 March 1983, US president Ronald Reagan announced the Strategic Defense Initiative, a research program into ballistic missile defense which he claimed would be "consistent with our obligations under the ABM Treaty". Reagan was wary of mutual deterrence with what he referred to as the "Evil Empire", and wanted to escape the traditional confines of mutual assured destruction. The project was a blow to Yuri Andropov's overtures for peace. Andropov said, "It is time Washington stopped thinking up one option after another in search of the best way of unleashing nuclear war in the hope of winning it. To do this is not just irresponsible. It is madness".
Regardless of the opposition, Reagan gave every indication that SDI would not be used as a bargaining chip and that the United States would do all in its power to build the system. The Soviets were alarmed because the Americans might have been able to make a nuclear first strike possible. In The Nuclear Predicament, Beckman claimed that one of the central goals of Soviet diplomacy was to terminate SDI. He went on to state that a surprise attack from the Americans would destroy much of the Soviet ICBM fleet, allowing SDI to defeat a "ragged" Soviet retaliatory response. The Soviets could not afford to ignore Reagan's new endeavour; therefore, their policy at the time was to enter negotiations with the Americans. By 1987, however, the USSR withdrew its opposition, concluding the SDI posed no threat and scientifically "would never work".
SDI research went ahead, although it did not achieve the hoped-for result. SDI research was cut back following the end of Reagan's presidency, and in 1995 it was reiterated in a presidential joint statement that "missile defense systems may be deployed... [that] will not pose a realistic threat to the strategic nuclear force of the other side and will not be tested to... [create] that capability." This was reaffirmed in 1997.