AS what used to be called the Soviet Union splits apart and recombines into new combinations, cold-war Europe has become ancient history faster than anyone could have predicted. The phrase "Iron Curtain" today sounds muffled and bygone, like "Treaty of Utrecht," "Austro-Hungarian Empire," or "Napoleonic Wars."Back when the Berlin Wall fell, the boldest American analysts said that maybe, possibly, East and West Germany might start moving toward reunification around 1992 or so. The Soviet Union itself would never fall apart - its people were resigned to their lot and it had no history of democracy. Reform would have to come top-down, from the Kremlin. Now United States officials are busy trying to keep up with a multiplication of nations that they didn't foresee. Where will the fragmentation end? In the ex-Soviet Union, the Russian Republic itself contains 16 smaller autonomous regions, many of them restive. Yugoslavia is clearly no longer a nation, but neither yet are its parts. Czechoslovakia may have an admirable president, Vaclev Havel, but its Slovakia half may soon push for independence. Whom to recognize? Whom to encourage? "We're seeing a situation where Europe is redefining itself," sighs one knowledgeable State Department official. The daily diplomatic problems and pressures facing the US are exemplified by the curious split personality of the Soviet Embassy in Washington. Over the last several weeks the embassy has housed, in effect, two ambassadors: titular Ambassador Viktor Komplektov and Counselor-Minister Andrei Kolosovsky, who is functioning as an ambassador from the Russian Republic. Counselor Kolosovsky has been urging US officials to recognize Mr. Yeltsin's new commonwealth as a legitimate government. His nominal boss, Ambassador Komplektov, has been urging continued support of the embattled Mikhail Gorbachev and what remains of central authority. This conflicting arrangement was set up with the full knowledge and concurrence of both factions back in Moscow, says a Soviet source. If the center finally collapses, as seems increasingly likely as of this writing, perhaps Kolosovsky wi ll emerge as full ambassador. The emergence of the Commonwealth of Independent States (CIS) seems as if it will settle the issue of the Kremlin's and Gorbachev's fate, at least for the moment. To this point, the Bush administration has held back from fully embracing the new CIS, whether from lingering respect for perestroika's architect or a desire to see the dust settle before making a choice. In calling for an international conference on aid to the Soviet Union, the White House has tried to take the initiative on at least the humanitarian front. But there are still lingering political concerns about the future of the Soviet confederation: Russia and the Ukraine, for instance, seem to have some important differences about the degree of integration the new arrangement will involve. Russia itself is far from a monolith, and still faces the prospect of crumbling around the edges. "We really don't know what the Russian Republic is going to be," says a State Department official. Almost 20 percent of Russia's population lives in 16 autonomous republics, all of which have replicas in miniature of Boris Yeltsin's own breakaway political system. The parliaments of these republics-within-a-republic have all declared their local laws sovereign over those of Russia. Some have called for independence, and a few of those really mean it. The tiny Caucasus mountain region of Checheno-Ingushetia, for instance, has so far successfully resisted Yeltsin's attempts to assert his authority there. Yakutia, a vast slice of the Russian Far East, has laid claim to control its natural resources. "The real reason we have this immense pressure for independence is that people feel cheated by the old imperial structure," says Adrian Karatnycky, research director of the AFL-CIO international department and an expert on Soviet nationalities. Outside the old Soviet borders, Yugoslavia is perhaps the thorniest problem in Europe for the US. Continued declarations that Yugoslav republics should settle differences peacefully, coupled with reliance on the European Commission to mediate disputes, hasn't worked. Yet only "reluctantly and slowly," according to a US source, has the Bush administration come to the conclusion that the old Yugoslav federation is no longer operative. US policy on which new nations it will welcome and recognize is based on five points first outlined by Secretary of State Baker in September: respect for democratic principles; safeguarding of human rights, including those of minorities; commitment to changing borders only through peaceful means; and adherence to international and arms control agreements. On Thursday Baker listed Russia, the Ukraine, Kazakhstan, Armenia, and Kirghizistan as showing clear signs of meeting these principles. He singled out the Republic of Georgia, where communism has been replaced by an increasingly authoritarian government, as an entity "undeserving of our acceptance and support."