Early last month, in what might have seemed just another document review by just another government lawyer, the Defense Department asked former New York assistant district attorney Philip Kunsberg to read the fine print of one of the most important arms control agreements between the United States and Soviet Union -- the Antiballistic Missile Treaty of 1972.

Kunsberg, 35, whose background includes battles against pornographers and the Mafia but no arms control experience, spent less than a week studying secret records of the ABM negotiations. His conclusions, outlined in a 19-page report, have triggered an uproar in Washington and Moscow by leading to a reinterpretation of the treaty -- and reversing the legal positions of four U.S. presidents, including President Reagan -- to allow nearly unlimited development and testing of the components of Reagan's Strategic Defense Initiative, the so-called "Star Wars" shield against nuclear missiles.

What seemed at first glance to be an obscure issue of treaty interpretation -- intriguing mostly to lawyers -- has become a central question affecting the future of U.S. arms policy, Star Wars and the summit meeting next month between Reagan and Soviet leader Mikhail Gorbachev.

The Kunsberg conclusions, modified and embraced as administration policy, strikes at the heart of the Soviet Union's preeminent objective at the summit and arms talks under way in Geneva -- curbing the U.S. strategic defense program. The Soviets have described such curbs as their essential requirement for any superpower agreement in limiting offensive nuclear missiles.

Kunsberg's unlikely entrance into global power politics began last spring when he was hired as a Pentagon staff lawyer by Fred C. Ikle, undersecretary of defense for policy, and Richard N. Perle, assistant secretary for international security policy, both Star Wars advocates. One of his first tasks in early May was to analyze the ABM Treaty to gauge its impact on strategic defenses.

The traditional interpretation of the treaty accepted by the United States since its signing and ratification by the Senate during the Nixon administration -- and repeated in reports to Congress by every administration since -- had been that the pact bans "testing" and "development" of mobile and space-based ABM weapons based on "new physical principles" such as laser beams, directed energy weapons and the other exotic technology of Star Wars.

The ban is not flatly stated in the treaty but is based on the cumulative evidence of several phrases and statements, some ambiguous. Reagan, in unveiling his SDI dream on March 23, 1983, had promised that Star Wars would be conducted "consistent with our obligations" under the ABM Treaty.

Nevertheless, trying to fit Star Wars within the treaty restrictions had become increasingly problematic as the U.S. program moved closer to a phase requiring technical testing of potential components. In April the administration offered an interpretation of the treaty that distressed arms control advocates. In a report to Congress, the Pentagon argued that major Star Wars tests planned in coming years would involve only restricted "subcomponents" of systems, or would be tested against satellites in space instead of incoming rockets, and thus not violate ABM stipulations.

As Kunsberg began his review, however, he was "astonished by the rather large gap between what the treaty said and what was attributed to it," according to a source familiar with Kunsberg's thinking. His suggestion that the treaty might allow even more room for Star Wars was followed by a request from Perle in early September to have Kunsberg examine more closely the record of treaty negotiations in order to evaluate the backing for the widely accepted restrictions on "exotic" ABM systems such as those now contemplated for U.S. strategic defenses.

There was no doubt, according to Kunsberg's subsequent report, which included extracts of the negotiating record, that the United States had sought a tight ban on "exotic" ABM systems of the future except for those (unlike Star Wars) in a fixed land-based mode. But Kunsberg concluded that the Soviets had never agreed and, to the contrary, consistently rejected the broad ban advocated by the United States. Kunsberg did not interview any of the U.S. negotiators, but drew his conclusions from the written record.

Perle "almost fell off the chair" when presented with Kunsberg's report, the assistant secretary later remarked. Suddenly most of the problems posed for future Star Wars "testing" and "development" by the ABM Treaty seemed of doubtful validity, and Kunsberg's analysis even questioned whether Star Wars "deployment" would be curbed by the treaty.

Perle and the Defense Department lost no time in presenting their dramatic new findings about the ABM Treaty restrictions to an interagency committee. The new Pentagon position seized the attention of officials concerned with arms negotiation policy, who quickly understood the large-scale significance of the "new interpretaton" for SDI, the ABM Treaty and U.S.-Soviet relations on the eve of the summit.

Secretary of State George P. Shultz called in the newly installed State Department legal adviser, Abraham D. Sofaer, 47, who came to Washington this summer after six years as a U.S. District Court judge in New York City. Charged by Shultz with studying the issue, Sofaer began his review of the negotiating record as maintained in Foggy Bottom. In addition, he held lengthy discussions with Paul H. Nitze, the administration's senior arms control adviser and the only senior member of the 1972 ABM negotiating team still in government.

Sofaer found the Pentagon report to be more opinion than analysis but, after several all-night bouts of study, reached a conclusion close to that of the Defense Department. He concluded that the U.S. negotiating team had tried -- but failed -- to convince the Soviet Union to ban future "devices" which might be invented to do the antimissile job.

Sofaer also concluded that while the treaty does not ban testing or development of space-based futuristic ABM systems, unlike the Pentagon he concluded it does ban their "deployment."

Sofaer's conclusions were reported to Nitze and Shultz in a series of memos, the most extensive and authoritative of them on Thursday, Oct. 3.

The following day, Oct. 4, the administration's high-level Special Arms Control Policy Group (SAC-G) met to discuss the issue at the White House under chairmanship of White House national security affairs adviser Robert C. McFarlane. During this meeting, according to informed sources, Pentagon representatives accepted Sofaer's position, rather than their broader one.

The discussion moved toward acceptance of the Sofaer position as new government policy, several sources said, though no formal agreement was reached. There are conflicting reports about whether there was a consensus around the table.

Toward the close of the late-afternoon meeting, McFarlane ordered a new position paper written to discuss a new legal position on the ABM Treaty that relieves the United States of most of the restrictions on Star Wars. At least some of those present expected to thrash out the issues further in connection with preparation of this paper.

All this had taken place in secret, without the knowledge of even well-informed people outside government. Last month, when the former counsel to the ABM Treaty negotiations, John D. Rhinelander, was asked by a journalist about rumors of a new treaty interpretation, he advised his caller that "this is too ridiculous to take seriously" and promptly forgot about it.

The shift became public in a way that is still a puzzle to many in government, on a "Meet the Press" interview with McFarlane Oct. 6. In response to a question from Marvin Kalb which did not mention the ABM Treaty, McFarlane volunteered the view that "testing" and "development" of ABMs based on "new physical concepts" is "approved and authorized by the treaty" rather than banned.

This startling pronouncement by a high official, almost a 180-degree reversal of the longstanding U.S. position on the treaty, was a shock to the ABM Treaty's negotiators and other arms control advocates, to U.S. allies in Europe and arms control-minded members of Congress.

When Gerard C. Smith, chief negotiator of the 1972 treaty and a well-respected veteran of six administrations, learned that McFarlane's pronouncement was a serious expression of policy rather than a mistake, he denounced the new interpretation as erroneous and said it would "make a dead letter" of the ABM Treaty.

In an interview, Smith questioned "the whole idea of searching the negotiating record" for the meaning of the treaty. He said that, while some of the final language was "not the best," it was clear to him and other U.S. negotiators that the Soviets explicitly agreed to tight limits on future "exotic" ABM systems such as those envisaged in the Strategic Defense Initiative.

Smith said the most important negotiations on this point took place in a "working group" headed by Sidney Greybeal for the United States and a then little-known Soviet official named Viktor Karpov, who went on to be chief strategic arms negotiator for Moscow in 1979-83 is chief Soviet negotiator at the current offensive and space arms talks in Geneva.

Greybeal was reported by Smith and others to be certain that the Soviets clearly agreed in the working group to ban "testing" and "development" of space-based and other "mobile" ABMs based on exotic technology. (Greybeal declined to be interviewed on this matter.) Prof. Albert Carnesale, a dean of the John F. Kennedy School of Politics at Harvard University, who was a member of the Greybeal-Karpov working group, said in an interview that his understanding at the end of the negotiations was that space-based exotic weapons could not be tested or developed.

"Any other interpretation doesn't make sense," Carnesale said.

Retired Lt. Gen. Royal B. Allison, who was the senior military official on the U.S. ABM negotiating delegation, said in an interview he was deeply involved in the discussions of this issue because the Joint Chiefs of Staff were determined to leave room for "research and research-testing" in case "exotic" ABMs such as lasers could someday be developed.

"Nowhere did I understand that we retained the right to development and full-scale testing of new systems," Allison said. He added, "I didn't have any doubt in my mind as to what the Soviets had approved. If there had been any doubt you can be sure we would have announced it."

Of the members of the former U.S. negotiating team, only Nitze was consulted by Kunsberg, Sofaer or other U.S. officials in connection with the reinterpretation of the ABM Treaty. Sofaer's position is that the U.S. negotiators may have sincerely believed they had a Soviet agreement, but that the record is devoid of evidence to that effect.

Sofaer has also said, though, that the record is incomplete: new documents were still coming to light even after his report that galvanized the government.

Nitze's position, as a senior official of the Reagan administration, was the most important to the internal deliberations. Nitze declined to make a public statement of what he believed the treaty to provide when it was signed in 1972 or what he has come to believe now.

However, sources close to him said Nitze is now convinced that the Soviet delegation rejected U.S. efforts to ban "testing" and "development" of exotic ABMs, though they did agree that "deployment" would have to be negotiated by the two sides.

From the beginning of the internal debate six weeks ago, Nitze is reported to have taken the position that, legalities aside, the most vital job of the administration was to "keep faith" with the interpretation of the treaty previously presented to U.S. allies, Congress and the public. Nitze appears to have been the driving force behind Shultz' proposal, ultimately accepted by Reagan in a showdown White House meeting Oct. 11, that the "new" ABM interpretation be accepted by the administraton as a matter of law, but that the "old" ABM interpretation restricting Star Wars testing and development be retained as a matter of administration policy.

The final decision, signed by Reagan according to official sources, did not say how long he would continue to observe the "old interpretation." Since it is now judged to be valid only as a matter of presidential policy, rather than as treaty law, it could be presumably be abandoned whenever convenient.

The Soviet Union, which had little to say as the "new interpretation" of the ABM Treaty became a matter of public debate in Washington, began to warm to the subject late last week.

The chief of the Soviet general staff, Marshal Sergei F. Akhromeyev, accused the Reagan administration of "deliberate deceit" in reinterpreting the ABM Treaty. In a Pravda article, Akrohmeyev charged McFarlane with having "distorted the essence of the treaty" by "trying to substantiate the lawfulness of experiments" that would lead to a space-based ABM.

Among the several mysteries remaining about the administration's reinterpretation of the 1972 treaty is why the issue came to the fore now, when the Star Wars program is still about five years away from breaking though the bounds of the treaty as previously interpreted, at least as the treaty's prohibitions have been applied by the Reagan administration.

Some of the advocates of the new interpretation said the timing was essentially a coincidence caused by the "fresh look" at the evidence by the two new government lawyers. Others, including a senior White House official, said the issue arose now because "people are staking out their ground" for the long term before the Reagan-Gorbachev summit.

Some in the administration are "very nervous" that Reagan might agree to some restraints on SDI in his meeting with Gorbachev and are moving to forestall such a possibility by reinterpreting the ABM Treaty now, this official said.

On the other, and at the moment the losing, side, the official said, are those who still hope for compromise with Gorbachev on SDI as the essential building block of a broad U.S.-Soviet agreement.