Ronald and Mikhail: the summer of their content - and a new detente. Elizabeth Pond traces the course of the superpowers' warming relations, a post-World War II journey that at times found Washington and Moscow struggling to keep the cold war from becoming radioactively hot. Ms. Pond, now on leave of absence, was the Monitor's Bonn correspondent. This news analysis is adapted from the revised (June 1988) edition of her book, ``From Yaroslavsky Station: Russia Perceived''

THE wary camaraderie that blossomed at the Moscow summit between the odd couple of Ronald Reagan and Mikhail Gorbachev was either inexplicable or foreordained. It is one of history's great reversals. And yet it is the utterly logical consequence of four decades of the balance of terror. Even as the prospect of nuclear annihilation has made general war unthinkable, so has this prospect made unthinkable for the leaders of the United States and Soviet Union the kind of indulgent political hostility that might erupt into nuclear war.

It was no smooth evolution that led to the present expectation of an arms control agreement cutting the superpowers' strategic arsenals by 50 percent. It was instead a turbulent, dialectical process. The Soviets needed first to discover that their economy was in crisis and that they could not press a military advantage in the third world in the '70s without triggering a profound reaction in the United States. The Americans needed first to discover that they could not restore American nuclear superiority through technical wizardry in space in the '80s.

Technologically, the nuclear standoff of the post-World War II era depended on a perversely benign ``deterrence,'' or prevention of war, based on mutually vulnerable populations and mutually invulnerable weapons. There was no known defense against the awesome destructiveness of nuclear weapons. Thus, any nuclear war would yield no winners, but only joint victims of a suicide pact.

Under this existential threat the two superpowers practice unwonted caution on that plot of earth that was most important to both: Europe. By fall of 1988, the standoff will have given Western Europe more than 43 years of peace, its longest reprieve in 2,000 years of bloody history.

In those 43 years, however, there were Berlin, Cuban, and Mideast crises that made the superpowers wonder if their cold war was going to turn radioactively hot. After several such scares, they moved to reduce volatility and enhance stability in the Antiballistic Missile Treaty (SALT) of 1972.

But then each side sought to break out of the stalemate - Moscow by exporting Cuban soldiers to Africa and Soviet soldiers to Afghanistan, Washington by a ``star wars'' program that might restore US nuclear superiority of the '50s and '60s.

Ironically, star wars and its failure formed the essential catalyst - in the atmosphere of the new Soviet sense of domestic crisis - that stimulated the US and the Soviet Union to reaccept the stalemate and seek the more durable accommodation that now looks possible.

Star wars was President Reagan's impetuous dream, launched in a speech in 1983, of reversing the four decades of the balance of terror. His Strategic Defense Initiative - SDI to the cognoscente, star wars to the laymen - aimed to erect in space the elusive defense that could at long last intercept attacking missiles. If it worked, it would for the first time in two generations end that utter vulnerability of populations and aggregate invulnerability of nuclear weapons, to make people secure and weapons insecure.

The catch was that to do this it had to operate all but perfectly. Failures of 25 percent, or 10 percent, or even 1 percent, in stopping 10,000 incoming warheads, each with 20 or 50 times the blast of that baby A-bomb at Hiroshima, would still mean the destruction of civilization.

On the face of it, the mission was impossible. Yet Mr. Reagan's single speech altered the political landscape. It powerfully focused minds in Moscow on the precarious balance of opportunities and risks in the nuclear era.

If the Soviets had not already felt themselves to be in crisis, the gleam of star wars in Reagan's eye might not have alarmed them as much as it did. But SDI aggravated their existing fears: that the Soviet Union might fall even further behind in the whole technological race, that the arms rivalry in space might absorb so many resources that it would abort economic modernization, that by the 21st century the nation might become a has-been as a superpower.

Soviet officials claimed from the beginning that they could and would counter any US deployment of SDI by simply overwhelming it with a cheaper proliferation of warheads - and rationally they believed their own words. Emotionally, however, they were nagged by doubts. What if the US deployed a rudimentary SDI system that would not be very effective defensively in thwarting any massive first strike by the Soviet Union - but would be highly effective offensively in guaranteeing a crippling US first strike on Soviet missiles by deflecting the Soviet Union's subsequent ``ragged retaliation''? And where would Moscow get the money to compete in space?

As Soviet strategists mulled over the unequal contest, a funny thing happened. All the deterrent verities about stability and assured second strike and the destabilizing, trigger-happy nature of the strategic defense that US arms controllers had preached in the '70s suddenly made a great deal more sense to the Soviets of the '80s. In the end they began propounding the old US precepts that Moscow had shucked off in veering toward a more Soviet view of the nuclear competition. More and more in the negotiations, Moscow's tacit aims converged with those of classical Western arms control even as Reagan's aims diverged from them.

Thus, if the Soviets sought to rule out a US first-strike capability, they had to sacrifice their own first-strike capability. If they sought to constrain the American SDI for the rest of the century, they had to constrain their own heavy missiles for the same period - and champion the strict interpretation of the 1972 Antiballistic Missile Treaty that they had originally evaded and Reagan now wished to evade.

If they sought US predictability and gradualism in the arms race to ensure that crisis outlays for weapons would not rob their civilian economy, they had to forfeit any nasty surprises of their own. If they were going to deter the US, they had to let the US deter them. And to consummate the bargain that SDI and perestroika made them want so badly, they had to accept two principles that had hitherto been taboo: intrusive on-site inspection and (at least in the Intermediate-Range Nuclear Forces [INF] Treaty the two sides signed last December and ratified in May) asymmetrical destruction of weapons.

It took a man of Mikhail Gorbachev's daring, speed, and tactical shrewdness to bring the Soviet Union to such uncharacteristic acts.

His innovations started in one of the more discretionary areas of foreign relations, policy toward the third world. Relations with Asia and Africa (and, more recently, Latin America) had undergone nine previous swings of the pendulum in Soviet history, and the 10th was already in process when Mr. Gorbachev stepped in, so he found it easy just to add momentum.

The swing was from left to right, away from infatuation with Marxist-Leninist fanatics who tended to ruin their impoverished economies and then expect the Soviet Union to rescue them, to flirtation with the more successful, if more bourgeois, nations lurching toward prosperity and regional importance.

Soviet disenchantment with distant self-styled Marxist-Leninists reflected pure national interest. The price of imperium was exorbitant, running to $40 billion or $45 billion a year in the early '80s, in Western estimates. Ingratitude was rife, as the Guineas and Ghanas kept following the path of Yugoslavia and China and Egypt in turning against their patron. And various clients that remained loyal also remained embarrassing: By abusing their economies so egregiously, Mozambique and especially Ethiopia were giving socialism a bad name in the rest of the third world. The Indias, Mexicos, and Brazils held much more promise.

WHEN new opportunities beckoned, then, the Russians cast a gimlet eye on them. They did not meddle in troubled Haiti or the Philippines or South Africa. They did not foment Baluchi separatism in Pakistan. They did not embrace the prickly Muam-mar Qaddafi more closely. They moved circumspectly in the Gulf. And even though they relished the nuisance value of the Sandinistas in Washington, they kept aid to Nicaragua at a modest $250 million over the first half of the '80s, a level below US support for guerrillas in Afghanistan.

Afghanistan, with its heavy Soviet investment of blood and treasure, was the most vexing Soviet dilemma in the third world. Yet even here Gorbachev drew the logical conclusions about what he called a ``bleeding wound.'' After eight long years of doing no more than holding the administrative centers of towns against the encroaching mujahideen, Moscow moved to retreat, without insisting that the successor government in Kabul be pro-Soviet. With this decision Gorbachev became the first Soviet leader ever to put a higher priority on cutting losses than on maintaining the record of the irreversible tide of history toward Soviet-style socialism. The Soviet superpower, too, it seemed, could have its Vietnam. Soviet withdrawal impressed the US, and suggested that Moscow might not again risk a new d'etente for ephemeral victories in developing countries as it had done in the '70s.

Viscerally, retrenchment was hardest on the fringe of empire. Oddly, it was far easier in the apocalyptic but disembodied world of nuclear weapons.

Under Gorbachev, military policy quickly became a function of his overriding economic priority. His pet perestroika, or economic overhaul, would have no chance if an unconstrained arms race turned into a black hole for rubles. Yet Reagan had demonstrated that the US would not be outspent in defense. And NATO had demonstrated in deploying Euromissiles in the early '80s that a Trotskyite appeal to antinuclear peace movements in the West over the heads of their elected leaders would not work. The only way to get the breathing space the Soviet Union needed was to negotiate with Western governments in general and with the US in particular.

Expressing this conclusion more elegantly, Gorbachev began stressing within a year of taking office the need for ``new thinking'' in security and awareness of the ``interdependence'' of states in the nuclear era. Unilateral security is no longer possible, he told the 27th Communist Party Congress in early 1986; it must be mutual or it will not endure. Nor can military means or even military parity ensure security in the absence of political accommodation and deference to the ``genuine national interests'' of the US.

Later that year, Gorbachev stopped voicing the old Soviet demand for ``equality and equal security'' - a formula that usually meant insistence on a right to Soviet power equal to the combined forces of the US, Western Europe, and China - and began substituting for it the far less ominous goal of ``reasonable sufficiency.'' ``Sufficiency'' remained a vague yardstick.

But Gorbachev specified that he had in mind arsenals appreciably smaller than current US and Soviet levels. And he was talking about a deceleration in military spending. Gone was the standard open-ended promise to give the generals ``everything necessary to reliably defend the homeland.'' Indeed, Gorbachev's new party program sounded a parsimonious note in calling only for a modest ``level that excludes strategic superiority by the forces of imperialism.''

GORBACHEV encountered less overt opposition to his ``new thinking'' than Leonid Brezhnev had engendered earlier with his more timid innovations about the impossibility of achieving nuclear superiority or victory.

No Politburo member made dark allusions to the high cost that d'etente was exacting in breaching secrecy with on-site inspection, condoning treason in the emigration of 400,000 able bodies, eroding discipline by releasing dissidents and stopping the jamming of foreign radio broadcasts, and undermining social mobilization by blurring the image of enemy encirclement. The new consensus was too strong - not necessarily on the specifics of Gorbachev's program, but on the general thesis that drastic measures of some sort were needed after the immobilism and drift of the previous decade.

The military hierarchy - again in contrast to the later Brezhnev years - apparently shared in the new consensus. Its acquiescence derived in part from the conviction that only immediate rejuvenation of the civilian economy could guarantee the base for better weapons later - and that only a breathing space abroad could make that regeneration feasible.

Besides, the quirks of superpower relations made those old American axioms of deterrence and stability now appear not as fetters on Soviet missiles, but as fetters on the threatening US SDI program. The new Soviet stress on the importance of both sides' maintenance of a retaliatory nuclear capability began to sound reminiscent of the kind of stability and finite limit to nuclear procurement defined by US Secretary of Defense Robert McNamara 20 years earlier as ``mutual assured destruction.''

Nor did the Russians, this time around, boast as Brezhnev had done in the d'etente of the '70s that the ``correlation of forces'' was turning in their favor. They knew now that nuclear deterrence could not long survive new Cuban expeditions in Africa or virulent anti-Semitism in the Soviet Union. Grudgingly, with some irritation at Western meddling in their sovereign affairs, they increased Jewish emigration to facilitate the new d'etente, freed several hundred dissidents, and transferred the notorious psychiatric wards of the KGB secret police to the regular health service.

With at least a particle of consensus on a new course, then, Gorbachev gambled on Reagan, the declared archenemy of the communists but the man who, like Richard Nixon, could deliver the American right in ratification of arms control. The two superpowers negotiated the INF Treaty as a kind of test case, then plunged ahead with the crucial strategic talks.

Simultaneously, the American right discovered that its dream of restoring US nuclear superiority through SDI was a technological and financial pipe dream. The core of the right reaccepted the stalemate the right had denounced so vehemently before it had its own crack at breakout, and failed.

For his part, Reagan never acknowledged that his SDI program was far more effective as a catalyst to reevaluation of old thinking than as a nuclear shield. But he came to learn something of the Russians as human beings who touched him. He decided that the Soviet ``evil empire'' he had railed against belonged to a bygone era. He went to Moscow last month calling Gorbachev his friend. Perestroika was nurtured. For the rest of the century predictability and stability were at least given the benefit of the doubt.

The cold war, some veteran Washington observers began to hope, might even be over.