On Oct. 3, 1972, it seemed as though half the nuclear arms race had been halted. That is when a treaty between the United States and the Soviet Union limiting antiballistic missile systems went into force. In essence, as Henry A. Kissinger said at the time, the idea was to give offensive missiles ''a free ride to their target'' and thereby ensure that both nuclear powers retained their retaliatory force.
Today, though, many arms-control doubters - including some key Reagan officials - wonder whether the ABM treaty ought to be changed, if not scrapped.
They look at the continuing buildup in nuclear weapons on both sides (which the ABM treaty was supposed to slow), the big advances in technology since then, and the allegations that the USSR is violating the ABM treaty in fashioning a nationwide missile-defense system. They wonder if the 12-year-old treaty has not outlived its usefulness, if the US should not use its technological edge to defend against Soviet missiles.
In response, many nuclear strategists and former arms-control and defense officials have mounted a vigorous defense of the ABM treaty. They view President Reagan's controversial strategic defense initiative (''star wars'') as a direct threat to what some see as the most successful superpower agreement in the nuclear age.
''The American people are being misled into believing there is a magical solution to the nuclear predicament,'' says Gerard C. Smith, the Republican who negotiated the first US-USSR strategic arms agreement as well as the ABM treaty.
''A US 'star wars' effort will prompt a similar effort by the Soviets,'' says Ambassador Smith, and ''compel both sides to accelerate their race in offensive weapons, and increase the risk of nuclear war.''
The essence of the 1972 ABM agreement (and its 1974 protocol) is that the superpowers should be limited to a single defense system of no more than 100 interceptor missiles around the national capital or one ICBM (intercontinental ballistic missile) field. These defensive missiles may not have more than one warhead, nor may their launchers be rapidly reloadable or mobile. Both countries also agreed not to develop, test, or deploy sea-based, air-based, space-based, or mobile land-based ABM systems, although research in these areas is allowed.
The treaty was an acknowledgment of the overwhelming destructive force of nuclear weapons.
''It is a realpolitik approach, not an ideal one,'' says Sidney Drell, physicist and codirector of the Center for International Security and Arms Control at Stanford University. ''The ABM treaty is the formal recognition that mutual destruction could not be escaped if the superpowers were drawn by accident or design into nuclear war. ... It accepts deterrence as a present necessity and objective condition, not as an active threat which would be intolerable.''
Today's debate over missile defense - prompted by President Reagan's controversial speech last year - echoes the one heard in this country in the late 1960s. But there are several important reasons for its revival.
First, as even critics of the President's initiative acknowledge, there has been remarkable progress in those technologies (sensors, computers, directed energy, and ways to transport things into space) that could be part of an advanced defensive system.
Second, fears about the possibility of nuclear war - due in large measure to the lack of significant progress in limiting weapons of mass destruction - have heightened public interest in pursuing protective measures. Opinion surveys (including polls taken a few months before Reagan's ''star wars'' speech in March 1983 before the conservative Heritage Foundation) consistently show more than 80 percent of the public favoring strategic defense.
And third, there is mounting evidence that the Soviets may be positioning themselves to ''break out'' of the ABM treaty by deploying systems not allowed under the agreement. Among these is a large phased-array radar (which can track many targets at once), advanced mobile antiaircraft missiles that could possibly be used against other missiles as well, and ABM launchers that US intelligence sources suspect can be quickly reloaded.
The United States in the mid-1970s built its allowable ABM system (called Safeguard) around 150 Minuteman strategic nuclear missiles in North Dakota. But it was dismantled a few months later because of its high cost and the realization that Soviet missiles probably could penetrate it.
The Soviet Union has kept its Galosh missile defense facilities around Moscow and now is building an improved ABM-X-3 system with better interceptors and radars.
Critics of the President's strategic defense program are quick to point out that the US also may now be testing systems that encroach upon the ABM treaty. These include: Talon Gold, the pointing and tracking portion of a space-based laser; the Airborne Optical System, an infrared telescope carried aboard an aircraft to track and identify warheads above the atmosphere; and the electromagnetic rail gun, which accelerates projectiles to very high speeds.
Administration officials deny that any such advanced research is in violation of the ABM treaty.
''It is the policy of this administration that only experiments permitted by the ABM treaty will be conducted,'' asserts Lt. Gen. James A. Abrahamson, director of the Strategic Defense Initiative Organization.
''No sooner than the 1990s would we bump up against the ABM treaty in testing for development,'' says another Air Force officer who is currently working on strategic defense.
Part of the controversy is caused by some imprecision in ABM treaty language. US and Soviet negotiators could not settle on exact definitions of such terms as ''develop'' and ''ABM component.'' Pentagon officials and documents often describe planned ''demonstrations'' of subsystems designed to show the feasibility of a possible ballistic missile defense system.
''Supposed imprecision in treaty language is being exploited by both nations to provide a rationale for vigorously pursuing missile defense technologies while claiming adherence to the treaty's terms,'' states a report published in June by the National Campaign to Save the ABM treaty.
This recently formed group is headed by Ambassador Gerard Smith, and includes among its members Jimmy Carter, William Colby, George Kennan, Robert McNamara, Dean Rusk, Stansfield Turner, and Cyrus Vance.
Under the terms of the treaty, the two sides meet every five years to review the agreement. The next scheduled meeting will be in 1987. With six months' notice, either side can withdraw from the treaty ''if it decides that extraordinary events ... have jeopardized its supreme interests.''
It is very unlikely that the US administrations (assuming Reagan wins a second term) would make such a move, especially with a presidential election so near. But there are those urging US abrogation.
''The treaty to my mind expresses an obsolescent theory of deterrence,'' says Colin Gray, president of the National Institute for Public Policy and a member of the presidentially appointed General Advisory Commission on Arms Control and Disarmament.
Adds Dr. Gray: ''I really think that if the SDI (strategic defense initiative) fails, it's going to be on policy and strategy and arms-control grounds and not on technical grounds.'' And that is exactly what Gerard Smith and other ABM treaty supporters hope will happen.Next: An interview with the general who heads the Pentagon's strategic defense program.