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.
