IT WAS ALMOST 150 years ago that the celebrated traveler Alexis de Tocqueville predicted, with notable prescience given the realities of his time, that Russia and America were destined to be the great powers of the future. Today The Giants , as Richard Barnet calls them in the title of his new book, dominate the globe as two states never have before.

The destructive might of both countries - not to mention their vast economic resources - is daunting. Rome was truly a great imperial power and Britannia did once rule the waves. But neither could blow up the world in an instant. De Tocqueville foresaw a traditional superpower rivalry. Modern technology has raised the possibility of an apocalypse.

The Giants is a laudable effort to explain Soviet-American relations, the focal political issue of our day, to the general reader. It is an intelligent essay, written cogently without heavy jargon or polemic. The book's purpose is to place the events of the moment in a historical context so that Americans might find it easier to follow the bewildering ups and downs.

Barnet's emphasis, therefore, is on the background of what we know as detente and the Russians call razrydka (relaxation) - the accomodation of what are fundamentally hostile attitudes, in the respective leaderships, to changing needs.

Henry Kissinger made the term synonymous with the U.S.-Soviet ties in the 1970s, but its antecedents span the decades.

In the early 1930s, even before Washington had formally recognized the Kremlin, when the world capitalist economy was suffering from a depression, American business saw virtues in the Bolshevik market. The United States became a major commercial exporter to the Soviets. And in World War II and alliance of experience emerged from the tatters of the Nazi-Soviet pact and the rubble of Pearl Harbour to defeat the Axis.

And so it was in recent years that the parties reached a point where mutual benefit was in reducing thensions. The latter-day era of detente, Barnet writes, "was officially proclaimed when the American and Soviet elites acknowledged to each other that for pratical political purposes the countries were equal in destructive power."

This portrayal of two elites jockeying with each other - "the small group of politicians, diplomats, generals and businessmen on each side . . . changing their relationships in complex and curious ways" - is central to Barnet's perspective:

"What each society has produced, how each has spent its money, how much freedom and dissent has allowed, how much secrecy each has craved and how each has treated the rest of the world has been substantially determined by what a handful of men in the White House and Kremlin thought their counterparts were doing or were about to do."

To the extent that any such simple rubric can still be valid in an age of confused ideologies, Barnet is an American radical. After a spell in government in the early 1960s, he founded (with Marcus Raskin) the Institute for Policy Studies, a scholarly outpost for the post-Marxist left.

Barnet's radicalism means that he is suspicious of the conglomeration of power whether it be the multinational corporations (subject of his last book) or the bureaucratic tyranny of the Soviet Communist Party's highest echelons and the self-righteous imposition of Americanism on other lands by our own establishment.

For the purposes of The Giants , Barnet's approach is valuable. It enables him to asses U.S.-Soviet dealings without really approving one side or the other which is what few analysts can resist. Barnet sees deep-seated, long-standing fears on both sides reinforcing friction, an inevitable consequence of two messianic societies rubbing against each other. He sees as particularly strong the vested interest of both, military elites in having an adversary as an excuse for mobilizing defense.

"The official eschatolgy in both Washington and Moscow," Barnet argues, "still embraces the fantasy of mass conversion - the day, as Walter Lippmann once put it, when Soviet babies are born singing 'God Bless America' or, in Khrushchev's prophecy, when John F. Kennedy's grandchildren will live under communism. From the start of U.S.-Soviet relations, each side has tried to induce internal political change in the other out of a belief that its own security and power demanded it."

For at least the years of the Cold War, the prevailing American dogma was that Kremlin aggressivenes, ignorance and ideological bumptiousness were mainly to blame for superpower antagonisms. Barnet clearly advocates what a Russian dissident friend of mine in Moscow disapprovingly called Symmetry - the notion that responsibility for the problems should be apportioned.

Detente is not, however, a question of who has the predominance of virtue. It is a search for areas of agreement, a means of controlling confrontations, of evolving entanglements, as Helmut Sonnenfeldt, Kissinger's chief adviser on Soviet affairs (and hardly an apologist for the Kremlin) once said.

"The alternative," Sonnenfeldt gloomily observed, "is a policy of isolaton that will lead to war."