The ABM Treaty’s most famous line was that two sides were allowed — but the treaty’s real point was to forbid a third. Signed in Moscow on May 26, 1972 as the SALT I component on missile defense, the Anti-Ballistic Missile Treaty initially permitted each side two anti-missile deployment areas (reduced to one per side by a 1974 protocol), on the strategic theory that a robust nationwide defense would tempt a first strike and destabilize the nuclear balance. The treaty remained in force for thirty years, until the United States withdrew in 2002 to permit the missile defense programs the agreement had been written to prevent.
The strategic logic of the treaty is the key. A robust nationwide missile defense would tempt a country to launch a first strike, on the assumption that it could absorb the retaliatory blow. The other country, knowing this, would be driven to expand its own offensive forces to overwhelm the defense. As long as both sides were vulnerable to retaliation, neither had an incentive to start a nuclear war. This was mutual assured destruction (MAD), and the ABM Treaty was its legal embodiment. See the SALT treaties and arms control for context.
The treaty allowed each side initially to deploy ABM systems at two sites: one to defend the national capital, one to defend an ICBM field. Each site was limited to 100 interceptor missiles. The Soviet Union deployed the Galosh system around Moscow; the U.S. deployed a single Safeguard site at Grand Forks Air Force Base, North Dakota, operational only briefly in 1975. Verification relied on national technical means, primarily reconnaissance satellites, supplemented by the Standing Consultative Commission.
The treaty was challenged twice during its lifetime: by the U.S. Air Force’s interest in missile defense, culminating in Reagan’s Strategic Defense Initiative of March 23, 1983, and by Senate conservatives who argued for years that the U.S. should withdraw. Both challenges were deflected by administrations that judged the strategic costs of withdrawal higher than the costs of compliance.
The United States withdrew on December 13, 2001, with the withdrawal taking effect on June 13, 2002. The George W. Bush administration argued that the treaty was a Cold War relic, that proliferation required missile defense, and that the era of mutual vulnerability was no longer acceptable. The decision remains controversial.