NEWS ANALYSIS : Moscow Caught Flat-Footed by Crisis : Diplomacy: Once it would have reacted decisively to a breakup of Yugoslavia. Now it can only watch.

Through the long years of the Cold War, the Soviet Union saw the breakup of Yugoslavia, with its potential for civil war, foreign interference and escalation, as one of the most worrisome crises that East and West might face in Europe.

With that scenario now exploding into reality 500 miles from its border in one of Europe’s historic tinderboxes, the Soviet Union is clearly apprehensive but uncertain what it can do in a region that it once thought to be within its sphere of influence.

“We share the concern of other nations that this process of disintegration in Yugoslavia might affect the stability in Europe as a whole,” Vitaly I. Churkin, the Soviet Foreign Ministry’s chief spokesman, commented this week.

“We would not like to turn back to the 19th Century instead of entering the 21st Century. That would, of course, have the gravest consequences and, to a large extent, reduce the great achievements . . . that have brought peace and tranquillity on the European continent.”

The Soviet Foreign Ministry in a formal statement Friday evening backed the central government’s call in Belgrade for a three-month moratorium on all changes in Yugoslavia’s character, including the secession of any of its republics.

“The existence of a united, independent Yugoslavia is an important factor of stability in the Balkans and the whole of Europe,” the Soviet statement said, noting the world’s “deep anxiety” over the current conflict. “Assistance to and support for the Yugoslav people at this time of trial in their history is a requirement for constructive European and world politics.”

Yet, in a crucial test for both the “new world order” and the “European common home,” two concepts that President Mikhail S. Gorbachev has championed in Soviet foreign policy, Moscow appears to have been caught strangely flat-footed by the Yugoslav crisis, uncertain of what influence it has and how to use it.

“In the old days, we would have put our troops on alert, moved additional units up to air bases for deployment, probably reinforced the Warsaw Pact and made some pretty strong statements,” a veteran Soviet strategist commented, asking not to be quoted by name because of his government position. “Today, we have trouble drafting a statement. . . .

“Ironically, this is a crisis--the breakup of Yugoslavia with some republics going capitalist and seceding--that we had thought about, planned for and exercised for over three or four decades. . . . Intervention, by us and by the West, was always taken as possible and even probable.”

During the Cold War, the Balkan Peninsula was for Moscow an even likelier East-West flash point than the heavily fortified frontier between the two Germanys, according to Soviet foreign policy specialists, because of the Kremlin’s commitment to “defend the gains of socialism” throughout the world.

“Yugoslavia was always seen as inherently unstable, and thus the likelihood was always strong that one, two or even three republics would try to break away and go capitalist,” a former Kremlin adviser remarked. “And that is more or less what is happening now. . . .

“If the West intervened or if the socialist majority called for our assistance, we were prepared--in the old days--to respond, and we let everyone know it. . . . That commitment is gone, absolutely gone, and that is a measure of how things haved changed. We watch with concern, but we watch.”

Even three years ago, Yugoslavia’s stability was of such great sensitivity for Moscow that Soviet politicians and generals regularly visited Belgrade to assure themselves that the growing nationalism of the Serbs or the Croatians or the Slovenes posed no threat.

Churkin acknowledged that Moscow now has no ideas, no plans and no proposals beyond backing a call by the foreign ministers of the countries belonging to the Conference on Security and Cooperation in Europe for a solution based on “the democratic development, unity and territorial integrity of Yugoslavia.”

In its statement Friday evening, before the tentative accord was reported in Yugoslavia, the Soviet Foreign Ministry said that “any individual or collective international efforts should now be aimed at ensuring, in cooperation with the Yugoslav government, the necessary conditions for preserving Yugoslavia’s integrity and independence and helping the Yugoslav peoples ensure a reliable future for their country in a democratic and peaceful way.”

This and an earlier Foreign Ministry statement recalled the historic pledge by Gorbachev during a 1988 visit to Yugoslavia of nonintervention--an end to the so-called Brezhnev Doctrine.

Moscow had justified its intervention in Czechoslovakia in 1968, as in other East European countries earlier and, a decade later, in Afghanistan, as comradely assistance to defend socialism, an argument articulated by the late Soviet President Leonid I. Brezhnev.