For over two decades the Anti-Ballistic Missile (ABM) Treaty has been an important diplomatic tool for managing the strategic nuclear arms competition. The Treaty was signed by the USA and the USSR and entered into force in 1972. Amended in a Protocol in 1974, it is now in force for the USA and for Russia as the legal successor to the USSR. It obligates them not to undertake to build a nation-wide defence system against strategic ballistic missile attack and severely limits the development and deployment of permitted missile defences. Among other provisions, it prohibits giving air-defence missiles, radars or launchers the technical capability to counter strategic ballistic missiles or from testing them in a strategic ABM mode.

Ballistic missile defence (BMD) reappeared on the arms control agenda in 1993: at issue was the Clinton Administration's proposal to permit the testing and deployment of new advanced-capability theatre missile defence (TMD), or anti-tactical ballistic missile (ATBM), systems designed to defend US allies and US armed forces operating overseas. Critics have argued that these systems would have significant capabilities to intercept strategic ballistic missiles and that allowing deployment would create a serious loophole in the ABM Treaty.

TMD systems are not formally subject to the Treaty, which limits only strategic ABM systems. However, the threshold between strategic and theatre ballistic missiles is not technically clear-cut and the characteristics of strategic and non-strategic defences overlap. In November 1993 the USA initiated discussions with Russia at the Standing Consultative Commission (SCC) seeking to establish a demarcation between theatre and strategic missile defence systems based on demonstrated technical performance parameters. These discussions had stalled by the end of 1994. The Clinton Administration announced in early 1995 that the USA would proceed with the testing of a sophisticated new long-range TMD system, despite objections that doing so would be a violation of the ABM Treaty.

Both US and Russian advanced-capability TMD programmes and the proposed allowances in the ABM Treaty seem excessive in comparison to avowed current threats or hypothetical future threats. Even if agreement is reached in the SCC on revising the ABM Treaty, the testing and deployment of the systems currently under development threaten to undermine the integrity of the Treaty as one of the cornerstones of the post-cold war strategic nuclear balance. They would reduce the prospects for further nuclear disarmament by the USA and Russia and compel the other nuclear weapon states to reconsider their strategic force modernization plans. In addition, they would threaten eventually to weaken the non-proliferation constraints among the non-nuclear weapon states.

New provisions for more intrusive verification methods and transparency could ensure ABM Treaty compliance in line with the post-cold war logic of TMD advocates. The ability of the USA and Russia to cooperate in the development of theatre anti-missile technologies would be a good test of the validity of their `strategic partnership'.