WHEN NATIONAL Security Adviser Robert C. McFarlane suddenly informed those watching "Meet the Press" on NBC national television on Oct. 6, 1985, that the ABM Treaty didn't mean what it had been thought to mean for 13 years, he was informing not only them and the world at large but also everyone else in the U.S. government, including President Reagan.

McFarlane's sudden discovery was grounded in a quiet reinterpretation of the 1972 ABM Treaty that had involved a handful of people. The gist of the revised view was that although the language of the treaty plainly said that each side wouldn't "develop, test or deploy ABM systems or components which are . . . space-based," the administration could still develop and test space-based ABM systems -- if they were based on "new physical principles."

During the two years since McFarlane's revelation, the reinterpretation has spawned a bitter controversy. It has been denounced by all the principal ABM Treaty negotiators except Paul Nitze and declared to be unsubstantiated by a number of leading members of Congress, including conservative pro-SDI Sen. Sam Nunn. An uneasy stalemate between the administration and Congress now prevails and may come to a head soon, as the issue is tied both to current defense-appropriations legislation and to support of the prospective INF treaty.

Where did this new ABM interpretation come from? The answer is that it was the work of a small cadre of conservative activists who were intent on pushing the Reagan administration's political agenda. Interestingly enough, one of the early propagandists for the ABM reinterpretation -- a lawyer named Bretton G. Sciaroni -- also provided a controversial legal rationale used by the White House in the Iran-contra affair.

Virtually from the time of the signing of the ABM Treaty, some people in the Pentagon began considering ways to loosen its constraints. But the idea that the ABM Treaty did not mean what it seemed to say first surfaced in 1975 with the late Donald G. Brennan at the Hudson Institute. Brennan had opposed the ABM Treaty from the start and strongly advocated strategic missile defenses. He circulated his views in an exchange of letters with me and other experts on the treaty but never published or publicized his thinking. The same general idea was conceived independently by Abraham S. Becker and Wiliam R. Harris at the Rand Corp. in 1977 as they were studying possible Soviet circumvention for "breakout" from the ABM Treaty.

The election of Ronald Reagan gave Harris the chance he had been looking for. In early 1981, the hard-line transition team in charge of the Arms Control and Disarmament Agency -- James T. Hackett, Michael E. Pillsbury and Davis S. Sullivan -- hired Harris as a consultant to help prepare a study of Soviet arms-control violations. In the course of this work on ABM Treaty compliance, Harris raised the question of ABM Treaty interpretation. Although at that time nothing came of either the compliance or interpretation issues, both later resurfaced.

The next midwife of the reinterpretation was T. K. Jones, a deputy undersecretary of defense who is most widely known for his 1981 statement that "with enough shovels, everybody's going to make it" through a global nuclear war. In late 1984 and early 1985, he commissioned several studies to review the ABM Treaty and its negotiating history to see if there were any loopholes that would allow more leeway for the SDI program and to ascertain whether the United States was holding itself to a more narrow interpretation than the Soviet Union. When career government lawyers and other experts endorsed the traditional view of the treaty, Jones turned to Harris. He submitted his own analysis in early February 1985, prepared after only two weeks, opening the door to a broad interpretation of the treaty.

The issue went public on April 4 when the Heritage Foundation issued a report promoting the broad interpretation. The report, credited to an anonymous government official, was titled "U.S.-Soviet Arms Accords Are No Bar to Reagan's Strategic Defense Initiative." The identity of that "anonymous official" has long been a mystery. W. Bruce Weinrod, the director of research at Heritage who commissioned the report, says that because he is not a lawyer, he asked for help from a government attorney -- whom Weinrod still refuses to identify.

The covert author, it turns out, was Bretton Sciaroni. Sciaroni had served briefly with the interim ACDA team under Hackett, Pillsbury and Sullivan. In 1984, he became the counsel of the president's Intelligence Oversight Board. (As has been previously reported, Sciaroni qualified for a waiver for the political appointment to that position only after failing his bar examinations four times in his native California and the District of Columbia, before passing in Pennsylvania.)

Sciaroni's name surfaced in the Iran-contra investigation because of his role in September 1985 in rendering an offhand ruling that the NSC staff was exempt from the Boland Amendment barring any U.S. government official involved with intelligence activities from participation in providing military assistance to the Nicaraguan contras.

The new interpretation then emerged in official government arms-control policy circles in September 1985 as the result of a review prepared in the Defense Department. A young lawyer recently hired at Defense, Philip H. Kunsberg, had been asked in the spring of 1985 by Richard N. Perle, the assistant secretary of defense for international security, and Fred C. Ikle, undersecretary of defense for policy, to take a fresh look and see if there was more leeway in the ABM Treaty than had been recognized. Kunsberg's experience of record had been in combatting pornography and the Mafia in New York, as noted in press accounts. These earlier accounts didn't, however, disclose that he moved to Defense after five years in the office of the general counsel of the Central Intelligence Agency, Stanley Sporkin. Building on Harris' approach, Kunsberg prepared a 19-page report with a radically revised interpretation of the ABM Treaty.

The final midwife was was Abraham D. Sofaer, the State Department legal adviser, who was asked to review the legal validity of the new interpretation. It was also referred to the office of the general counsel of the Department of Defense and to the general counsel at ACDA, which had, earlier in the year, reconfirmed the traditional reinterpretation of the ABM Treaty. So had Nitze, who in an address on May 30, 1985 had publicly restated the traditional interpretation.

Sofaer turned the matter over to three young lawyers he had brought with him into government service a few weeks earlier. Sofaer's team found flaws in Kunsberg's analysis. But a hasty review of the negotiating record and consultation with Nitze, the only member of the Strategic Arms Limitation Talks (SALT) ABM Treaty negotiating team serving in the Reagan administration, led Sofaer to decide the record could be used to support the broader interpretation on development and testing of ABM systems using future technology.

Sofaer was informed by one of his staff researchers, William J. Sims III, that a memorandum by the assistant general counsel of the Department of Defense, John H. McNeill, rejected the proposed reinterpretation; that Thomas Graham Jr., the general counsel of ACDA, also rejected the interpretation, and that Sims himself had concluded that only the traditional interpretation was justified. Sofaer did not, however, report any of this in his memorandum of Oct. 3, 1985, to Secretary of State George Shultz and Nitze endorsing the new "broad" interpretation.

The author of a Senate Republican minority report on the ABM reinterpretation prepared early in 1987 was, no one will be surprised to learn, William R. Harris. On July 30, six conservative Republican senators (James A. McClure, Jesse Helms, Steven Symms, Malcolm Wallop, Orrin Hatch and Chris Hecht) sent a letter to the president recommending that Harris be appointed assistant director of the Arms Control and Disarmanent Agency. Among the key advisers on the matter to Senate Republicans are Michael Pillsbury and David Sullivan. It's a small world.

Raymond Garthoff is a senior fellow at the Brookings Institution and author of "Policy vs. the Law: The Reinterpretation of the ABM Treaty," which will be published Tuesday. He was one of the principal negotiators of the ABM Treaty.