Putting, as de'tente demands, the best face on Soviet behavior, we can say that glasnost has glitches, as current troubles with the Tartars show. But the truth is, those troubles are only the most recent recrudescence of a perennial Soviet problem -- ''the nationalities question'' -- that sets a severe limit on the scope of glasnost.

Tartar leaders recently were expelled from Moscow, where a few hundred Tartars were mounting minuscule protests, demanding that their original homeland in the Crimean peninsula be recognized as autonomous. In 1944, 250,000 Tartars were deported to central Asia (perhaps 100,000 died) as punishment for alleged collaboration with German invaders.

The Kremlin's disproportionate response to the recent protest included accusations that U.S. diplomats had worked ''to inspire nationalistic manifestations.'' Understandably, such manifestations rank high on the long Kremlin list of ''antisocial actions.''

The Soviet Union is run by a minority -- Russians -- that is declining, numerically, relative to many of the Soviet Union's other captive nationalities. Like most such minorities that govern resentful groups, the Russians are regarded as arrogant. They do indeed despise many other ethnic groups, including the Tartars, as ''Asiatic.''

Soviet leaders often denounce this attitude. On Oct. 6, 1922, Lenin was too troubled by a toothache to attend a Central Committee meeting, but he sent a note: ''I declare war to the death on Great-Russian Chauvinism. I shall eat it with all my healthy teeth as soon as I get rid of this damned tooth.'' He promised that the presidency of a crucial body ''should go in turn to a Russian, a Ukrainian, a Georgian, and so forth.'' But it would take more than 100 ''and so forths'' to cover all the unmeltable ethnic groups scattered across the Soviet Union's 11 time zones.

Besides, the essence of the Soviet state was and is ''democratic centralism'' -- control by a party organized from above and run from the political center, Moscow. This principle is incompatible with any local autonomy.

But the pretense is otherwise. In New York in 1960, Khrushchev said: The Soviet regime guarantees to all national minorities an unabridged right of secession, and the fact that no minority has exercised that right proves that no minority is aggrieved. (In the late 1940s some people in Mongolia were just learning that the 1917 revolution had brought them into the Socialist motherland.)

There are today three kinds of nations. One nation, the United States, is defined entirely by assent to political principles of universal validity. Most nations are of a second kind. They evolved organically from the coalescence of religious, linguistic and cultural affinities. Then there are nations that are not really nations. They are aggregations of unreconciled groups hammered and held together by force. Yugoslavia and some African nations are in this category, as is the Soviet Union.

The United States is uniquely a nation (in Lincoln's precise phrase) ''dedicated to a proposition.'' To become, say, truly British is a complex and protracted process. Diverse millions of immigrants have become full-fledged Americans in a morning, by assenting to its proposition.

The Soviet regime fancies itself like the United States, founded on clear principles universally convincing to minds unclouded by ignorance. But Marxism is everything refuted by reality.

Marxism is a 19th-century pseudo-science that claims to lay bare the inner dynamic of history, a dynamic driven by economic conflict that changes with modes of production. Marxism, a ''science'' of conflict, cannot accommodate the most striking fact of the late 20th century, the fact that ethnic assertion, not class struggle, is a prevalent form of conflict, from Mecca to Moscow.

But, then, the hope of communist universality -- ''Workers of the World Unite!'' -- collapsed in August 1914, when socialists in the German Reichstag voted for credits to finance the war that was beginning. The solidarity of the international working class was a fiction. The particularities of life, the traditional affinities that are the glue of nations, were more powerful, more real than the categories spun by Karl Marx in the reading room of the British Museum.

The absence of such glue, the presence of powerful centrifugal forces within the Soviet Union and the fact that the regime's substitute for cultural homogeneity is a bankrupt ideology -- all these factors mean that the Soviet regime can never be more than an occupying power sustained by force. Thus glasnost can never be more than a carefully controlled Kremlin tactic to confuse the West and motivate Soviet masses with a tantalizing mirage of freedom just over a forever receding horizon.