Declarations by Mikhail Gorbachev that "reasonable sufficiency" is the basis for Soviet military requirements -- a departure from past policy -- underlie the arms-control positions that Gorbachev will bring to Washington and may foreshadow important shifts in Moscow's military forces, according to U.S. and Soviet experts.

This concept, which Gorbachev unveiled in early 1986, has been emphasized by the Soviet leader, his defense minister and a variety of official and semi-official commentators since this summer, although it has been little-discussed in the West.

Many U.S. officials are wary that the rhetorical shift in Soviet policy may be intended for political and propaganda benefit in the outside world rather than military application at home. There is no sign yet of significant change in the size or configuration of Soviet armed forces to reflect what appears to be more modest requirements for defense in both nuclear and conventional areas, according to officials on both sides.

But if the implied promise of large cutbacks and less threatening configurations under "reasonable sufficiency" is translated into fact, either through arms-control negotiations or unilateral Soviet reductions, the shifts could be historic. The massive and growing military power of the Soviet Union -- both real and perceived -- and its increasing global reach have been at the heart of East-West tensions during the last 40 years, often provoking new phases of military buildup in the West. A lessening of the Soviet military threat could in time transform, and perhaps even end, the cold war.

In some respects the redefinition of Soviet requirements is strikingly reminiscent of the Nixon administration's 1969 shift from a goal of "strategic superiority" in nuclear weapons to acceptance of "strategic sufficiency." The Nixon shift was not defined in detail, but the change justified the imposition of limitations on the U.S. strategic defense budget and the negotiation of arms-control agreements that permitted a Soviet advantage in some areas.

"We believe that armaments should be reduced to the level of reasonable sufficiency, that is, a level necessary for strictly defensive purposes," wrote Gorbachev in his newly published book, "Perestroika." "It is time the two military alliances amended their strategic concepts to gear them more to the aims of defense," Gorbachev wrote.

Gorbachev has expressed much the same idea many times in recent months. On May 29 the Soviet-dominated Warsaw Pact declared this to be a key element of its official doctrine, pledging to "strictly comply with the limits of sufficiency for defense, for repelling possible aggression."

The Warsaw Pact communique last summer called for consultations with NATO on the military doctrines of East and West in light of the new Soviet ideas. While NATO has not responded to this challenge, senior U.S. officials said the Atlantic alliance will soon accept such discussions on condition that they be a serious and limited conversation about specifics of force structures. These discussions are likely to take place next summer or fall in the context of new East-West negotiations about stability and conventional force reductions in Europe, the officials said.

Defense Minister Dmitri Yazov, writing in Pravda July 27, defined reasonable sufficiency in vague terms, saying that currently "it means precisely the magnitude of armed forces necessary to defend oneself against an attack from the outside," a definition that U.S. experts consider to be meaningless and to reflect the Soviet military's lack of enthusiasm for the change.

For the future, Yazov wrote, the Warsaw Pact proposes to employ the sufficiency principle to reduce forces East and West on a mutually agreed basis "to such a level where neither of the sides, while insuring its defense, has the forces or means enabling it to mount offensive operations."

This concept of "nonoffensive defense," which derives from theories in the West German peace movement, is believed to be the most controversial aspect of reasonable sufficiency within the Soviet military and, in the U.S. view, the least realistic in today's circumstances.

"Nothing could be more in conflict with reality" than the idea that Soviet ground forces are mainly defensive in their character, said Edward L. Warner III, senior defense analyst of the Rand Corp. and a student of the new Soviet concepts. "In the last decade the Soviets have developed the doctrine, organization and capability to fight an offensive war . . . a blitzkrieg war . . . especially in Europe," he said.

Lynn M. Hansen, assistant director of the Arms Control and Disarmament Agency, said his recent discussions in Moscow indicated a sharp difference of opinion about "nonoffensive defense" between Soviet diplomatic officials and a few military officials with quasipolitical jobs at the Central Committee of the Communist Party, on the one hand, and purely career military officers on the other hand. "The career military people don't buy it," Hansen said. A Soviet analyst of military affairs said much the same thing in a recent informal conversation about this aspect of doctrinal change.

So far, most Soviet statements on the concept of reasonable sufficiency have been general, with many more declarations from semi-official experts and publications than from Soviet officials. In an effort to provide more definition from an official source, the Soviet Foreign Ministry recently made available its director of policy planning, Lev Mendelevich, who has been deeply involved in the evolution of the concept.

Mendelevich, interviewed last month during a visit to Moscow by Secretary of State George P. Shultz, drew a clear distinction between the application of reasonable stability in the nuclear field and in the field of conventional (non-nuclear) forces.

In the nuclear arena, Mendelevich said, the concept ultimately calls for the complete elimination of nuclear weapons but, in the meantime, it describes a condition of "strategic stability." This condition, which has drawn the interest of Shultz and other senior U.S. officials, was defined by Mendelevich as "when each side retains the capability for a retaliatory {nuclear} strike, but neither side for a disarming first strike." This concept has long been advanced by western arms-control specialists, but was only lately embraced, even to this extent, by Soviets.

There is no official calculation of the level of nuclear force required for this condition, but U.S. experts said it would be well below the proposed cuts of up to 50 percent in U.S. and Soviet strategic nuclear forces currently under negotiation by the two nations.

A semi-official Soviet study of "Strategic Stability Under the Conditions of Radical Nuclear Arms Reductions" given to foreigners in Moscow this year concluded that about 600 mobile single-warhead intercontinental ballistic missiles on each side, a total of 1,200, would be sufficient to achieve strategic stability in the absence of antimissile defenses, which the Soviets consider destabilizing. This would be a cutback of about 95 percent from the roughly 25,000 land-based, submarine-based, sea-based and bomber-based warheads in the two strategic arsenals today.

The potential application of reasonable sufficiency to conventional armies, long the domain of considerable Soviet superiority in numbers of tanks, artillery and some other weapons, has aroused keen interest among U.S. and West European experts.

Mendelevich told The Washington Post that reasonable sufficiency in conventional forces relies on the concept of "balance" at lower levels and on less threatening deployments, including the removal of tanks and other offensive weapons from front-line areas.

"It is impossible to put these things into practice unilaterally. It may only be done through a process of agreement," said Mendelevich. The issue of whether military cutbacks could be unilateral, or must be negotiated, is under discussion in Soviet circles but without any consensus so far, according to Raymond L. Garthoff, senior fellow at Brookings Institution and an expert on Soviet affairs.

A key aspect of reasonable sufficiency -- which implies that major cutbacks can be made without endangering national security -- is the concept of "asymmetrical" reductions, under which the greatest reductions would be made by the side with the largest forces, which in most cases is the Soviet Union.

This principle was a crucial facet of the soon-to-be-signed intermediate-range nuclear forces (INF) treaty, which requires the Soviet Union to eliminate four to six times as many nuclear warheads as the United States, depending on the calculation being made. In the U.S. view, disproportionate cuts will be equally essential to the new negotiations on reductions in conventional forces in Europe, which are expected to begin next year.

According to the most recent edition of the authoritative "Military Balance," published this month by the International Institute of Strategic Studies, the Soviet-dominated Warsaw Pact has a preponderance in Europe of more than 2 to 1 in main battle tanks, 3 to 1 in artillery and mortars and 5 to 1 in surface-to-air missiles. A senior U.S. official said the NATO allies are nearing agreement in confidential talks on an objective of cutting back major weapons in Europe to equal levels, which would require massive Soviet cuts and few if any reductions by NATO.

It is uncertain to what extent the Soviets will accept such disproportionate cutbacks in conventional forces. But the Soviet willingness to speak publicly of "asymmetrical" reductions, and act decisively to agree to them in the case of INF, is a far-reaching change.