Soviets yielded most on road to summit. Superpower leadership on the line at summit. Reagan and Gorbachev approach their talks politically weakened and struggling with their nations' economies. Both are hoping for achievements that will restore some lost luster to their political standings at home.

President Ronald Reagan and Soviet leader Mikhail Gorbachev meet tomorrow on a record that shows - depending on one's point of view - either extraordinary readiness to compromise on the part of the Soviets or extraordinary skill and tenacity on the part of American negotiators. In the treaty eliminating intermediate-range nuclear (INF) missiles due to be signed tomorrow, it is Moscow that has yielded almost all of its original positions, has finally accepted intrusive on-site inspection and exchange of previously top-secret military data, and will destroy four times as many warheads as Washington will.

In the talks about space-based strategic defense (``Star Wars,'' Antiballistic Missile [ABM], or Ballistic Missile Defense [BMD]), it is Moscow that has come around and condoned the bulk of the American Strategic Defense Initiative testing planned for the next decade and has withdrawn its demand for a veto on SDI deployment after that decade.

In the deep, 50 percent cuts that both sides now endorse in the Strategic Arms Reduction Talks (START), the compromises are more balanced.

A chart in today's Monitor (Page 18) tracks the concessions in these major negotiations and looks at the differences that remain to be resolved before a START accord could become the most important arms control agreement ever to be signed in the nuclear era. Dates for changes in Soviet positions refer to the formal talks in Geneva.

General/ideological issues:

The perverse logic of the nuclear era - in which the destructive powers of the atomic bomb broke the age-old cycles of alternating leapfrogs between technological ``offense dominance'' and ``defense dominance'' in war - was profoundly unsettling to both superpowers.

Once the US no longer had a nuclear monopoly, an altogether new ``balance of terror'' was inaugurated in which each superpower could destroy the adversary's society, but neither could do so without itself committing suicide under the other's retaliation.

The Americans were the first to alter their perceptions and see the dreadful new weapons as having far less the traditional mission of ``war fighting'' than the rather novel one of ``deterrence.''

In the 1960s, US Defense Secretary Robert McNamara set a finite limit on the number of US missiles: the number needed to ensure a devastating American retaliatory strike on the Soviet Union after any initial Soviet attack. This ratio - which expressed a factual technological condition at least as much as any deliberate policy of mutual vulnerability - he called ``mutual assured destruction,'' or MAD.

It was on the premise of a continuing world of MAD that the US sought ``stability'' and ``predictability'' in negotiating the Strategic Arms Limitation (SALT) I and II Treaties in the 1970s. And it was on this premise that the US negotiated the 1972 Antiballistic Missile (ABM) Treaty, limiting strategic-missile defense to two (later one) localized ABM sites. The aim was to avoid upsetting the stability of the offensive standoff by tempting one side to think it could launch an attack and actually win a nuclear war without triggering that unacceptable retaliation.

The Nixon administration codified this reasoning, informally lowering its sights from the goal of ``strategic superiority'' in nuclear weapons to one of ``strategic sufficiency.'' At the same time, the Nixon administration sought to establish a framework of superpower relations in the d'etente of the 1970s in which neither side would press the other for ``unilateral advantage.''

Nixon's fellow Republicans, however, rejected this evolution as too dovish, and the 1980 platform for Mr. Reagan's election campaign reverted to the goal of ``military superiority'' and confrontation with the Soviets. The new feeling was that Moscow had exploited the d'etente of the 1970s to lull the West while the Soviets made military advances in Africa and Afghanistan.

Casting US military and political resurgence in the 1980s against Soviet industrial and technological crisis, the early Reagan administration then sought to wage something akin to economic war on the Soviets to bring them to their knees and force domestic change that would tame the Kremlin's adventurism abroad.

It took until midway through the Reagan administration for the belligerence to mellow into modest Nixon-type striving, not to overthrow the rival regime, but simply to regulate the continuing superpower competition and prevent it from boiling over.

The comparable adjustment to the harsh realities of the nuclear age was, if anything, more painful for the Soviets than for the Americans.

The Soviets preserved the old Russian obsession with military mass, military over-insurance, and a security so total that it inherently required insecurity for Russia's neighbors. And as good Leninists, they felt obligated to give the occasional military push to help the dialectics of history press onward to the inevitable world triumph of the communist system. The notion of a nuclear stalemate therefore was an anathema.

Nikita Khrushchev was the first to diverge from ideological orthodoxy in saying in 1956 that war was no longer inevitable between communist and capitalist states. Two more decades passed before Leonid Brezhnev took the next step in renouncing military superiority as a goal in 1977. And it took another decade for Mr. Gorbachev, in his ``new thinking,'' to preach the ``interdependence of states,'' even states of differing social systems.

(Gorbachev's ``new thinking'' also moved Moscow away from Brezhnev's penchant of the 1970s to exploit in the third world a perceived shift in the global ``correlation of forces'' in the Soviet Union's favor.)

In pursuing ``interdependence'' in the specifics of arms control, Gorbachev has broken new ground - first, in accepting on-site inspection, and second, in putting forward Soviet initiatives. In the INF Treaty he has accepted for the first time verification of production at a major Soviet missile plant by 30 to 40 resident American observers.

Moreover, the Soviets have been negotiating seriously throughout 1987. This contrasts with the 1970s, when the Soviets delayed resolving key SALT issues until then Foreign Minister Andrei Gromyko's famous ``last 20 minutes'' of summits, to maximize public pressure on the American president.

Striking, too, is Gorbachev's unprecedented willingness to scrap new SS-20s. Until now the Soviets - who resist getting rid of even obsolete tanks or artillery pieces that hinder their own troops - have adamantly opposed destroying any new weapons.

Intermediate-range weapons

In this category the US yielded none of its original positions, except in forfeiting the option of converting its Pershing 2s and cruises to other missiles. (The current opposition to the INF Treaty by the American right entails no argument that the US didn't get its way in the treaty, but rather a criticism of the original American negotiating position in 1981.)

By contrast, this week's summit treaty requires abandonment of several Soviet demands. The Soviets agreed for the first time in any arms control agreement to destroy weapons and to allow intrusive on-site inspection. They accepted as well unprecedented asymmetrical destruction of warheads and the principle of equal global numbers for the US and the Soviet Union (not only in Europe but worldwide, and without compensation for British and French nuclear forces).

In lesser issues, the Soviets also ceded points in not insisting that the superpowers retain 100 residual INF warheads (in Soviet Asia and the US); in accelerating full destruction of missiles from five to three years; and in accepting elimination of West Germany's INF missiles with US warheads by unilateral West German action rather than superpower treaty.

Strategic (space) defense

Mr. Reagan has always insisted that SDI is no bargaining chip. But he has not refused to talk about space defense with the Soviets. He has always wanted to discuss the relationship between strategic offense and defense to persuade the Soviets that defense is good, while offense is bad.

In 1986, Reagan indicated flexibility by offering continued compliance with the 1972 ABM Treaty for 7 years under certain conditions. At the summit in Reykjavik, Iceland, a year ago he increased the time of compliance to 10 years (though the US later scaled this back to a period ending in December 1994). And for this week's summit he has allowed ``non-withdrawal'' from the ABM Treaty to go on the agenda.

Gorbachev, for his part, finally acknowledged that some strategic defense is inevitable in coming years.

Initially, the Soviets claimed that not only development and testing of space-based SDI systems were illegitimate under the ABM Treaty, but also pure research (which the ABM Treaty never limited at all).

In 1986, however, the Soviets approved SDI tests in ``laboratories'' and accepted a potential end to ABM treaty restraints after a finite period of ``non-withdrawal'' from the ABM Treaty. In reducing at Reykjavik the term of non-withdrawal to 10 years, he made a further concession, since one decade roughly corresponded in any case to the early phase of the SDI program before any US deployment of space defense could begin. Such constraint would therefore delay the American program by only one or two years at most.

But there were two catches. For the decade of compliance, the Soviets insisted on ``strengthening'' the ABM Treaty, and their presentation of what this would entail went well beyond even the ``narrow'' interpretaton of the treaty as the issue was debated in the US. What Moscow wanted to do was to add new restrictions that would, for example, ban any SDI testing in space (and not just testing of full ABM components, as specified in the ABM Treaty).

The second catch at Reykjavik was the implicit Soviet veto on American SDI deployment after the decade of restraint.

In 1987, however, the Soviets moved further toward legitimizing the US SDI program, with a speed that surprised American negotiators.

In submitting their draft space treaty in July, the Soviets quietly dropped the Reykjavik demand for a veto on later SDI deployment and tacitly approved SDI testing in space by submitting a list of ``thresholds'' of technologies, specifying what would and would not be permissible in space testing.

While this list remained more restrictive than even the ``narrow'' interpretation of the ABM Treaty, that difference was largely removed (some, but not all, US negotiators believe) in September and October in additional Soviet offers and clarifications of the ``thresholds.''

In his pre-summit interview on NBC last week, Gorbachev further assured Reagan by saying that SDI was not a ``subject for negotiations'' and that the Soviets do not object to those parts of the SDI program that are in accordance with the ABM Treaty.

Strategic arms reduction talks

Here both sides are aiming for ``crisis stability,'' though both are reluctant to admit it. That is, they are trying to design nuclear weapons that have McNamara's MAD assurance of being able to survive an adversary's ``first strike'' and still retaliate. This state would enable leaders to avoid hair-trigger reactions in a crisis and wait out ambiguous signals of attack to assess real damage and significance before launching a retaliatory barrage.

Ironically both the Reagan administration and the Soviets are reluctant to acknowledge that crisis stability is their goal. What used to be the Reagan right has long condemned acquiescence in unavoidable nuclear stalemate - and codification of that stalemate by enhancing ``stability'' and ``predictability'' through arms control - as a failure of the political will to victory. The Soviets, too, have long condemned acquiescence in class-neutral stalemate as a failure of resolve to bring about the ultimate victory of communism.

Yet in practice the Soviet proposals are designed to minimize first-strike weapons in the US arsenal, while US proposals are designed to minimize vulnerable first-strike weapons in the Soviet arsenal. The current differences in numbers are within normal horse-trading range and no longer entail incompatible concepts.

For a year the US and Soviet positions in START have actually been very close to each other, with the exception of figures on ``sublimits'' of particular categories of warheads under the agreed overall limits.

The American expectation has always been that when a deal was close on space defense, the Soviets would move on the START sublimits.

This movement finally began in September, when the Soviets agreed to halve the number of warheads on their unique ``heavy'' missiles to 1,540 and not to increase them subsequently.