AMERICANS and the rest of the world are experiencing a wave of euphoria over recent events in the Soviet Union that is totally understandable.The hard-line opponents of change have been vanquished; the forces of reform have been legitimized and strengthened. The peoples of the Soviet Union have won a battle for freedom, although democracy in the Western sense is still a goal to be achieved. Mikhail Gorbachev, on whom the Bush administration set such stock, has been rescued from the clutches of the plotters, and though his political power has clearly been eroded, he has become a born-again reformer in partnership with the hero of the anti-coup movement, Boris Yeltsin. There are several positive aspects to what has happened. First, the Soviet Union has taken a giant step out of its dark past and is on the road - nobody knows how rocky or how long - to democracy. Second, with the pending decentralization of power, the Soviet Union is becoming smaller and less threatening. Its foreign policy will likely be far less expansionist. Third, its domestic problems will necessitate cutbacks in military spending. Though the Soviet Union will remain a formidable nuclear power, its capacity to project military power will be much reduced. Fourth, its economic policies have failed and will no longer beckon the socialist-inclined leaders of the third world. Such fractious states as Iraq and Libya and Vietnam, along with the Palestine Liberation Organization, cheered on the putschists who sought to overthrow Mr. Gorbachev. But for the rest of the third world, the Marxist game is up and the coup plotters have been discredited as would-be architects of a new world order. While all these developments should be cheered, there are a fair number of negatives to be pondered. First, the Soviet Union as we have known it is headed for disintegration - and therefore instability. There are some ugly tensions between republics, between various republics and the Russian center, and even within some republics themselves. Civil wars are not out of the question. Second, none of the political events of the past few weeks have in any way changed the stark economic crisis facing the Soviet people. There are widespread shortages. People are fed up. They are angry with their leaders. There may be food shortages and riots this winter. Third, the angry mood may permit a hunt for scapegoats that is a contradiction of the democratic direction in which the country is supposed to be heading. Mr. Yeltsin, for example, has not shrunk from taking action against newspapers and news organizations that offend him. Fourth, the dispersal of the huge Soviet Army is a problem. Is the Army to be split up, with units allocated to the seceding republics? And what of the nationwide command structure and support system on which the Army depends? The same kinds of questions revolve around the disposition of the Soviet Navy and Air Force. The allocation, control, and use of these units is something of acute interest to the West. Fifth, and particularly critical, is the disposition of the Soviet Union's nuclear weapons. Will some of the nuclear arsenal come under the control of individual republics? Moscow's leaders say they will not, that they will remain under control of the Russian Republic. That is something that remains to be worked out, and about which Western nations are intensely concerned. Sixth, there are uncertainties about the future leadership of whatever kind of nation the Soviet Union ultimately becomes. Gorbachev has wrought great change and the United States, for instance, gambled heavily on his survival. But in the long run it is the ongoing policy of a country, not the personality of an incumbent, that is important. With Gorbachev's star waning, the US has embraced Yeltsin, who in earlier times was dismissed as a tippler and an erratic politician. As important as Gorbachev and Yeltsin may be, in the end it is the course the people of Russia and its associated republics set for the country that will be decisive.