In 1975 the United States decided not to deploy an anti-ballistic missile system; since then, it has concentrated on research.

There were two reasons for this decision. Ballistic missile defences would, by defending cities, reduce the effectiveness of the policy of nuclear deterrence based on the doctrine of mutual assured destruction, which depended on holding an enemy's cities as hostages to prevent the enemy making a pre-emptive nuclear attack. The second reason was the negotiation of the 1972 treaty on the limitation of anti-ballistic missile systems (the ABM Treaty-see Arms Control and Disarmament). This treaty prohibits the deployment of ABM systems for the defence of the whole territory of the United States and the Union of Soviet Socialist Republics, but permits the deployment of ABMs around just one area in each country-either for the defence of the national capital, Washington, D.C. and Moscow, or for the defence of one ICBM silo complex each.

ABM Treaty and SALT

The ABM Treaty was meant to strengthen the policy of nuclear deterrence by mutual assured destruction, and to slow down the nuclear arms race between the two superpowers by preventing each side from reacting to the other side's deployment of ABM systems-by increasing the number of its own ICBMs in order to saturate the other side's ABM defences. The first Strategic Arms Limitation Treaty (SALT I) entered into force at the same time as the ABM Treaty. It limited the number of ICBM launchers and the number of ballistic missile launchers on modern submarines deployed by both sides, and was also meant to slow down the nuclear arms race.

Despite limiting the increase in the number of nuclear weapons in the Soviet and American arsenals, the ABM and SALT I treaties totally failed to prevent the next qualitative round in the nuclear arms race from occurring: namely, the improvement in the quality of strategic nuclear ballistic missiles by the development and deployment of multiple independently targetable re-entry vehicles (MIRVs).

The Americans decided not to deploy ABMs; after the construction of one out of four projected systems, code-named Safeguard, to protect ICBMs housed in silos at Grand Forks, North Dakota, the US Congress closed down the programme. The Soviet Union, however, deployed an ABM system of Galosh and Gazelle missiles around Moscow. It became operational in 1964 and is still in operation. One hundred ABMs are deployed, supported by early-warning radar and battle management systems.