Thewillingness of Mikhail Gorbachev to scrap the Kremlin's entire arsenal of shorter- and medium-range nuclear-tipped missiles as part of an arms agreement with the United States is viewed by arms control experts here as a dramatic symbol of the Soviet leader's impact on revising this Communist superpower's national security policy.

Aside from agreeing to get rid of billions of rubles worth of new triple-warhead SS20 missiles, the new Kremlin leadership has also publicly acknowledged that deploying them in the first place was, in hindsight, a strategic error.

The missile agreement, completed in Geneva this week and to be signed at a Washington summit in 10 days, is the result of a Kremlin assessment that the damage caused by its deployment of hundreds of new SS20s in the 1970s -- which provoked new western unity and matching deployments of U.S.-built missiles -- far outweighed the gains, Soviet arms control experts here said.

The new missile agreement, in effect, reflects two new accents in Soviet security policy, both associated with Gorbachev's stated goals. One is a projected, gradual move toward minimal levels of nuclear and conventional arms. The second is a new emphasis on subjecting strategic defense decisions to rigid cost-benefit analysis, in which potential political and economic costs are weighed against projected military benefits, the experts explained.

"We decided we had to be a lot smarter about what we do," said Valentin Falin, director of the official Novosti information service, about the Intermediate Nuclear Forces (INF) treaty.

Together with the priority Gorbachev has placed on limiting President Reagan's Star Wars program for an antimissile defense, the more flexible stance being taken by Soviet arms negotiators and the more hard-nosed approach to cost-effectiveness are regarded by experts here as the key results of Gorbachev's two years of playing an active role in national security strategy.

Gorbachev clearly has raised the consciousness of western nations and their arms experts with his view that both superpowers can remain secure with vastly reduced arsenals of ocean-spanning strategic nuclear missiles and bombers, especially after he proposed deep cuts in nuclear weapons at the October 1986 Reykjavik summit.

He also has given unusual exposure to Kremlin brainstorming on national security policy, which usually is a rigidly held secret, and some policies have taken surprising turns.

The Soviet leader, for example, declared a unilateral ban on underground nuclear testing in August 1985, and extended it four times before finally dropping it 19 months later, apparently because of military objections.

Soviet officials have buttressed their public call for drastic weapons reductions with the argument that only "reasonable sufficiency" in conventional and nuclear arsenals on both sides is needed.

In a recent article in the official weekly New Times, arms specialist Vitaly Zhurkin defined this. In nuclear terms, he said, it means "sufficient to repel any aggressor but insufficient to conduct offensive operations."

In conventional terms, sufficiency means "ensuring an adequate defense potential so that the aggressor should not be able to count on a local blitzkrieg, or on escalating such a conflict with impunity," he said.

Soviet and western specialists are still groping for the levels of weaponry that constitute an adequate defense, however, particularly in the area of conventional arms, where Soviet Bloc forces hold a considerable numerical advantage and where sharp cutbacks are crucial to Moscow's efforts to reduce defense spending.

During his tenure in office, Gorbachev's national security policy has been regarded as a matter of trial and error, with the intermediate-range missiles treaty emerging as the first successful venture.

Asked in an interview to define the Kremlin leader's security policy, Soviet arms specialist Viktor Karpov said: "We are for weapons reductions -- conventional, nuclear, chemical." Besides abolition of all nuclear weapons, "the overall aim is to reduce to levels that would render a conventional attack impossible on both sides," added Karpov, who heads the Foreign Ministry's disarmament agency.

Karpov said the INF treaty should give an impetus to other arms negotiations that Moscow hopes to seal with the Reagan administration, including an accord to cut long-range missiles on both sides by 50 percent, an agreement on steps toward a ban on nuclear testing, an agreement to cut chemical weapons and an agreement in principle to reduce conventional arms.

The strategy, defined by other Soviet officials as a move toward "minimal sufficiency" in nuclear and conventional weaponry, exists more on paper than in practice, however.

Besides its 19-month ban on nuclear testing, Moscow seems reluctant to take unilateral moves toward "minimal sufficiency."

There is no evidence of a cutback in defense spending or cutbacks in weapons programs under Gorbachev's leadership.

Gorbachev's security policy started with a review of earlier decisions.

Under the scrutiny of party specialists, a number of missions

launched by Gorbachev's predecessors have taken a bashing, including the 1979 decision to send Soviet troops into Afghanistan to prop up a wobbly communist government and the heavy investments in military aid to Syria, Libya and other allies in the Third World.

The stockpiling of nuclear weapons orchestrated by U.S. and Soviet leaders in the 1970s and 1980s has come under heaviest attack, even though these weapons consume a small percentage of defense spending in both countries.

Worldwide elimination of nuclear weapons, advocated publicly by Gorbachev, is widely viewed as unlikely, however.

One obstacle is strong objections raised publicly in the West, particularly by British and French leaders, who strongly advocate the nuclear deterrent. Some Soviet hard-liners, too, favor nuclear deterrence.

"Nuclear weapons are the only thing that gives the U.S.S.R. superpower status," one western arms specialist in Moscow said. "Without them, it would be a Third World economy."

Another obstacle to further denuclearization in Soviet eyes is President Reagan's plan to build a space defense shield against nuclear missiles.

The Strategic Defense Initiative (SDI), or Star Wars, is viewed as a major threat to Soviet security since, if successful, it would stop Soviet nuclear missiles from reaching American soil in retaliation for an American first strike. It also poses a threat to Soviet economic reform, since any effort to respond by developing a Soviet nuclear defense system would drain resources needed for overhauling civilian industries. Thus, Gorbachev has linked cuts in strategic arsenals on both sides to some restrictions in the Star Wars program.

Despite the attack against heavy-handed military decisions, an unusual coalition of Soviet defense officials has rallied behind the move for deep cuts in nuclear arsenals, indicating strong support of Gorbachev's policy. Newly appointed Defense Minister Dmitri Yazov publicly supported the INF and strategic treaties in a speech here Nov. 7, for instance. And Army Chief of Staff Sergei Akhromeyev played a key role in advancing the Geneva negotiations for the intermediate-range missiles treaty.

The public support of senior Soviet military officials for Gorbachev's disarmament campaign has perplexed western arms analysts, who speculated that the high-profile Soviet strategic concessions are balanced by compromises made between party and military officials behind the scenes.

The shift in the Soviet stance on medium-range missiles since Leonid Brezhnev's rule offers a rare insight into a Kremlin cost effectiveness study in the making.

In one of its most controversial decisions, the party leadership under Brezhnev began the SS20 deployments in 1976. Despite angry objections from the West, the deployments continued for five years. By 1981, in Soviet Europe alone, 243 SS20s were in place. In Soviet Asia, at least 170 more would eventually be stationed.

The response from NATO, apparently not anticipated by the Kremlin, cost Moscow dearly. In London, Bonn and Paris, governments that had displayed a certain sympathy to Soviet politics turned more conservative. Across Europe, peace movements some sympathetic to Moscow eventually fizzled like punctured balloons.

With hardened resolve, NATO began in 1983 deployments across Western Europe of two new weapons systems -- cruise missiles and Pershing IIs, a newly designed single warhead missile, with a speed and accuracy that still daunts Kremlin military strategists.

In an unusual public acknowledgement of a mistake in Soviet security planning, a senior Soviet official recently suggested that deployment of SS20s was unneccessary and hinted that it had been forced by a heavy-handed military.

"The effective development of our technology rather than political analysis influenced the adoption of some decisions," Deputy Foreign Minister Alexander Bessmertnykh said in an interview published here.

In an interview, Soviet arms expert Karpov identified the elimination of the Pershings, along with the fresh impulse given to other arms talks, as the main Soviet interest in rushing the INF treaty to completion.

Although Soviet officials do not say so publicly, their national security policy also is aimed in part at lessening the military ties between the United States and Western Europe.

Next: "Reasonable sufficiency," just talk?