ON SUCCESSIVE Sundays Soviet armed units have shot their way into public buildings in the Baltics, killing 17 of the defenders and raising so-far unrebutted apprehensions that Mikhail Gorbachev is throttling reform. Yesterday Mr. Gorbachev again insisted that as a general proposition this was not so. But although he did not at once impose presidential rule on the democratically elected governments of the Baltics, a step that surely would have stirred their wrath, he made plain he intends to make centralist rule prevail. Some of his earlier admirers, Soviet and foreign, had hoped he would use the lingering element of ambiguity in the Baltic crackdown -- were the troops firing at Moscow's order or not? -- to slip into a more conciliatory mode. He did not.

The conclusion is becoming inescapable that Mr. Gorbachev has already gone as far as he will go with reform and that he is in the process of accommodating those elements in the society, including himself, who feel that the Communist Party, using the army, must take these political, economic and ethnic matters back into hand. But given the expectations that his own earlier policies stirred, there is a real question whether he can do this without prompting further massive unrest. Challenge is especially likely on the part of the republic governments. They constitute the chief counter to his own increasingly hard-line centralist rule, and they have taken his assault on Baltic sovereignty as a test run for a later assault on their own. If the Gorbachev part of reform is reversible, the republics and many individuals are determined that their part of reform is not. This is, of course, a formula for civil war.

To many people in the West, the tanks in Vilnius and the "Black Berets" in Riga are a bad dream. It still seems almost counter-intuitive that the supposedly courageous and imaginative Gorbachev would lose his nerve and become ready to sacrifice his country's desperate need for reform to satisfy his and his party comrades' deep-in-the-bone craving for power.

But the West must deal with the real situation unfolding before its eyes. It must study the international agenda to see what can still be saved and built on by cooperation with Moscow, which has come out for continuity in foreign policy. But it cannot conceivably become a partner in any latter-day Kremlin domestic agenda of rebuilding socialism and the unitary imperial state. Perhaps some part of the assistance intended for Moscow can be diverted to the republics or local bodies. The democrats in the country need to be encouraged in every possible way.