ON Nov. 19, Eduard A. Shevardnadze was reappointed as "Minister of External Relations of the USSR." Two days later, Russian President Boris N. Yeltsin was greeted in Germany by Chancellor Helmut Kohl with the full protocol honors accorded to the head of a sovereign state.On Nov. 27, American government officials announced that the United States would extend full diplomatic recognition to the Ukraine after its Dec. 1 referendum on independence. But two days later, George Bush indicated that he still hoped to maintain a close partnership with the Soviet central government of Mikhail S. Gorbachev in Moscow. What's wrong with this picture? Mr. Yeltsin, as his central role in the Chechen-Ingush secession crisis indicates, already wields more-effective power inside the former Soviet Union than Mr. Gorbachev. The impending international recognition of the Ukraine almost certainly dooms any hope of keeping a union together over the long term. As Yeltsin's predominance grows, he is unlikely to relinquish the conduct of the former Soviet Union's external relations to Gorbachev and Mr. Shevardnadze indefinitely. If the external world treats Yelts in like the head of a sovereign state, he will soon start acting that way in the international arena. One of the first places this may occur is in the United Nations Security Council. The Soviet Union's influence in the UN derives not from its rapidly vanishing international power, but from its formal position as one of five permanent Security Council members, with power to veto substantive UN activity. In September, former Soviet Foreign Minister Boris Pankin suggested that every Soviet republic could be admitted to the UN General Assembly, but that a single representative of a "Union of Sovereign States" should continue to exercise the USSR's permanent Security Council veto. Yet Article 4 of the UN Charter states that UN membership is accorded only to "states." Before long, some important international issue will arrive on the Security Council agenda. Perhaps it will be the creation of an unprecedented UN peacekeeping operation inside a still-legally sovereign Yugoslavia - a sensitive subject for a still-legally sovereign USSR that may be going down a similar road. Perhaps an ambitious despot in a region of less-than-vital interest to the West will launch a cross-border invasion, to test whether Operation Desert Storm was really about new world order or oil. Or perhaps the Ukraine, which was granted a "Potemkin UN seat" in order to get Stalin to sign on, will actually begin to exercise its membership by seeking UN mediation in its disputes with either the Russian or Soviet central governments. Gorbachev and Yeltsin may be in complete agreement on all these issues about how to instruct Yulii Vorontsov, the permanent representative of the USSR to the UN. But maybe not. What if Russian Foreign Minister Andrei Kozyrev, a young rising star, publicly challenges Shevardnadze over "the Soviet position" in the Security Council? And what if Yeltsin, irresistibly tempted by the opportunity to gain a formal voice in the affairs of the world, sends his own UN ambassador to New York, claiming that Mr. Vorontsov represents a state that no longer exists and that Russia should inherit the USSR's permanent Security Council seat? Even if the union dissolves so completely that an international consensus emerges in favor of Russia taking over the Soviet position at the UN, some transitional difficulties may still ensue. Article 23 of the Charter says that "the Union of Soviet Socialist Republics" shall serve as one of the five permanent members. Since no one would claim that Russia and the USSR are essentially the same state, it is difficult to envision how such a change in the Security Council's composition could occur without an amendment to the Charter itself. And since the Charter's framers made no provision for the disappearance of a state, it is equally difficult to envision who could legally cast the Soviet vote on such an amendment other than the permanent representative of the USSR. What if Gorbachev and Shevardnadze decide to exercise what is perhaps their last instrument of power by instructing Vorontsov to veto the proposed amendment? The Charter has never been substantively modified in its 46-year history. It is not inconceivable that a third of the General Assembly's members might coalesce to block any change in the former Soviet Union's UN representation unless other long-simmering representation issues are at least considered at the same time. World leaders have long complained about the anachronistic composition of the Security Council, and have discussed creating new permanent seats for Germany and Japan, large developing nations such as India and Brazil, the poor "South" as a whole, or even the European Community. Others have suggested transforming the General Assembly into a true global legislature, by converting to a weighted voting system based not solely on one nation-one vote, but on populations and contributions to UN finances as well . In addition, protracted Soviet political instability may remind the international community that any "new world order" centered around today's UN Charter stands utterly hostage to the prevailing political winds in places like Moscow and Beijing. Surely some have recognized that if some sort of Soviet coup had been successfully undertaken in the fall of 1990, there would have been no possibility of using the UN as the central mechanism for confronting Saddam Hussein, for assembling a broad international coalition, and for giving the economic and military instruments employed against Saddam the force of international law. So the breakup of the USSR may even provide the opportunity for some modification of the veto, so that collective UN action mig ht be undertaken when conflicts prevail among the great powers. The protracted conflict between the US and the Soviet Union, after all, is why the UN was essentially irrelevant to international affairs for the first four decades of its existence. The Soviet Union's disintegration could provide the catalyst for transforming the UN into a third-generation world organization, one better suited for confronting the complex global problems of the post-cold-war world.