The empire strikes back. A concern among Soviet policymakers is that the defeat in Afghanistan will trigger defiance in Poland, in Romania, or at home.

MIKHAIL GORBACHEV's drive to forge the Soviet state into a modern society marks the decline of the contiguous and internal Soviet empire. The internal nationalities conquered by the Bolsheviks and the Eastern European satellites secured by Stalin are all beginning to test the slack in the system created by glasnost. Conservative Kremlin planners fear that isolated incidents of unrest or acts of Soviet restraint could trigger trouble elsewhere. The Soviets now face labor unrest in Poland, ethnic rivalry between Armenia and Azerbaijan, and military humiliation in Afghanistan; these could act as catalysts for unrest in the Soviet Union and East Europe.

In this brave new world of perestroika, communist rule - until now the immovable object of this century - has collided with the concept of self-determination, the irresistible force of our time. Nationalist revolt has struck behind the Iron Curtain and fundamental changes are on the horizon.

Traditionally, the Soviets have proudly sponsored anticolonial movements in the third world while clamping the lid on any stirring sense of nationhood within their own country or sphere of influence. But the very decolonization process sought by Lenin for the empires of the West is now under way inside the hidden empires of the East.

Mr. Gorbachev's gerontocratic predecessors deployed tanks and troops to put down displays of defiance in Eastern Europe. After the Soviet invasion of Czechoslovakia, Leonid Brezhnev outlined a Draconian doctrine of socialist intervention. Gorbachev's new approach to Eastern Europe includes a pledge not to use force against other socialist states. Yet the acute problems between the Soviets and their satellites - economic dislocation, generational leadership change, and resurgent nationalism - will severely test Gorbachev's pledge of noninterference.

Soviet empire problems do not end in Eastern Europe; they reach inside the USSR. The Soviets are the inheritors of a domestic patchwork of nationalities that have chafed under Russian rule since the time of the czars. To the west of Moscow, the Baltic states seethe under Soviet domination, ever resentful of the pact between Hitler and Stalin which consigned their independence to the dusty pages of bourgeois histories.

Far south from the Kremlin corridors of power are the suddenly explosive communities of Orthodox Armenians and Islamic Azerbaijani in the Caucasus. Following the border east are the Muslim republics of Central Asia, where populations and religious fervor are multiplying. These nationalities are attempting to ride the domestic wave of perestroika toward greater autonomy, and in some cases independence.

No Soviet leader would willingly preside over the complete separation of Eastern Europe from the Soviet Union or the dismemberment of the internal empire.

Yet the contradictions in Gorbachev's call for openness and the prime directive to maintain order will be most acutely felt in Soviet relations with the socialist commonwealth. Currently, disgruntled Czechs and dissatisfied Armenians look to Gorbachev as a savior, while conservatives in Moscow and in Eastern Europe are fearful that he will set off a chain of ethnic disturbances and nationalist uprisings that will shake the very foundations of Soviet power.

Gorbachev faces the ultimate prospect of a domino-theory-like process sweeping through the Soviet empire, in which Soviet concessions toward one group will be viewed as a vulnerability and an opportunity by another. A perceptible concern among Soviet policymakers is that the defeat of the Red Army in Afghanistan will trigger defiance in Poland, in Romania, or at home. Gorbachev would then have to choose between sending in the tanks or not, between alienating the ethnic hopeful or the Kremlin hard-liners. Either choice could mark the end of his ambitious campaign to ``give socialism a second wind.''

The Soviet dominance of Eastern Europe and control of a domestic empire of separate nationalities exist as modern anachronisms in an age of national liberation and self-determination. It is hard to see how the Soviets can forever put off the process of empire-shedding faced by all great European powers.

In this election year, Americans have been hotly debating the purported decline of the American empire. Yet, the stuff of the modern American empire - competitive exports, the size of the national debt, and a handful of foreign bases - pales in comparison with the challenges of the Soviet empire.

The signs of American decline are subtle and reversible, but the Soviet predicament is dangerous and severe. By withdrawing from Afghanistan, Gorbachev has sent shock waves through the socialist world with a signal that resistance to Soviet rule can win in the end. Will the message reverberate throughout the Soviet empire?

Kurt M. Campbell is assistant director of the Center for Science and International Affairs at the Kennedy School of Government, Harvard University.