THE SOVIET INVASION of Afghanistan, combined with Congress' coolness toward SALT II, have brought relations between the United States and the U.S.S.R. to a virtual standstill. Neither President Carter nor Chairman Brezhnev trusts the other. Scientific exchanges are in abeyance, tourism has plummeted, and trade is all but nonexistent. And at the very moment when channels of communications are being cut, the number of points of pontential conflict multiply.

As tension mounts, it is the more important to know what the Soviets are up to. Is their global offensive rooted in defensive motives, as some have claimed? Are they reckless opportunists seeking to compensate abroad for shortcomings at home? Or is the U.S.S.R. really a conservative power, erring as much on the side of caution as excess? That there are responsible people on both sides of such questions testifies as much to the complexity of the U.S.S.R. today as to our own confusion.

Under the circumstances, the appearance of these two books -- one by a gifted British journalist and the other by a team of American Sovietologists -- is particularly timely. Both seek answers to the same questions, the former through an extensive tour of the southern republics of the growing flood of new publications and studies from the Soviet Union itselt. The conclusions of both are far more nuanced, and hence useful, than much of our recent public discussion.

Nora Beloff served as Moscow correspondent for the Observer of London and returned after a long absence to update her impressions, especially on the ethnic patchwork of the Caucasus and the Ukraine. Like countless Europeans over the centuries for presenting her observations. It was a happy choice. Beloff combines a keen eye with a feisty temperament. To keep her Intourist guides honest, for example, she brought along a copy of the famous 1914 Baedeker in her bag. Let anyone exaggerate the achievements of Soviet efforts and Beloff has her finger on the pre-revolutionary situation, as described by the teutonic Michelins of old.

Like many British intellectuals of her generation, she is not one to flinch from underscoring the Soviet Union's shortcomings. Yet she is fair. Though she was interrogated at the border and illegally denied access to the British consulate, she did not permit that incident to distort her vision. As a result, hers is a book worth reading.

According to Beloff, Soviet society is profoundly conservative, deeply insecure not because it has failed to accomplish much over the past decades but because its accomplishments fall so far short of its boastful claims. One suspects that these exaggerated claims, even more than actual Soviet policies, are the real object of her scorn. Yet Beloff is not among those western observers who have convinced themselves that the Soviet empire is seething with ethnic unrest that threatens to rip it asunder. Quite the contrary. Local consciousness may be on the rise but, as she rightly observes, millions of "ethnics" in the U.S.S.R. are linked to other groups besides their own by ties of blood and experience. The apparent passivity of Soviet Muslims in the face of Khomeini's Iran bears out her point.

The Soviet Union Since Stalin is a more substantial work, the product of a conference of scholars held at Indiana University about the same time that Beloff was bumping over the roads of Russia's South. It is possibly the only monument anywhere to the 25th anniversary of Stalin's death. Were he to read this book, Josef Vissarionovich would turn over in his grave, and not just because of the generous use of terms like "elite articulation" and "asymptotic" to describe the Soviet motherland. Like Beloff, the main theme running through the various contributions in the volume is the conservatism of the U.S.S.R. today, the sense of a lost future.

Political reformism is seen as foundering or dead, the 1977 "Brezhnev" Constitution is described as a "triumph for conservatism"; the economy is depicted as being unable to satisfy middle-class expectations either now or in the future; and Soviet foreign policy is represented as still clinging to the tradition of cautious opportunism set by Stalin. To this extent, the USSR would appear to be mired in old patterns and either unable or unwilling to evolve new ones.

But this is scarcely the whole story. It would be more accurate to characterize the Soviet Union as being in the grip of stasis than of stagnation. For every force for conservatism there exists not one but many counterforces, each seeking change in several directions at once. It is this condition of uneasy equilibrium that accounts for the absence of overall motion in Soviet development today. Caught like a rotor between opposing poles of a magnet, Leonid Brezhnev has at least prevented any one tendency from running completely unchecked. Hence his popularity and hence, also the prevailing sense of gnawing unease.

To the credit of the contributions to The Soviet Union Today, this perspective, too, is incorporated in the book. Stephen F. Cohen's "The Friends and Foes of Change" sets the tone, and nearly every other essay identifies the web of contradictory threads that are woven into the gray fabric of Brezhnevian Russia. On a domestic level, there may be little of women's liberation, but other forces in Soviet society are making family life there "less repressive and more egalitarian." Official literature may seem bland, but there are real counterforces not just in the self-identified "dissidents" but also in the same "official" writers, in provocative works that are only now becoming known in this country. And if Stalinist currents are still evident in the foreign policy of the U.S.S.R., party theoreticians in Moscow have at least scrapped the paranoid doctrine of "capitalist encirclement" and Stalin's (as well as Karl Marx's) dogmatic belief in the inevitability of war.

Reviewing these two interesting studies, one cannot escape the conclusion that we are witnessing the end of an era in the U.S.S.R., an era in which the perennial forces of bureaucratism and anarchism, rationalism and nationalism, cosmopolitanism and xenophobia, have been held in reasonable balance by a regime that wishes at all cost to make an outright choice. In this sense the Brezhnev era has been at least as pragmatic as it has been conservative. The problem is that the blend of practicality and caution that has characterized the past decade has not solved any of the outstanding problems on the Kremlin's agenda. Given this, it is entirely possible that future American leaders may look back with nostalgia at the relative stability of Soviet internal politics in the era of SALT II and Afghanistan.