THE defeat of the coup was a decisive moment in Soviet history. The plotters wanted to turn back the clock on reform and prevent the signing of a new all-union treaty that would shift power from the central government to the republics. The coup failed. Its demise could have a constructive legacy.The Soviet Union is a different place today than it was on Aug. 18. For the first time in Soviet history, the people acted to oppose those trying to determine their future. They took affairs into their own hands and came out into the streets in support of freedom. Their action was critical to the turn of events. The role played by the Soviet people gives great hope for the eventual success of the reform process. In the aftermath of the coup attempt, the situation in the Soviet Union is in flux. It will remain so for some time. Nevertheless, the opportunities to advance the process of political and economic reform are enormous. The situation is being transformed almost daily. The devolution of power from the central government to the republics has accelerated. At least 10 republics have now declared independence, though not all of these republics may wish to eventually secede from the union. Boris Yeltsin, the freely elected president of the Russian Republic, is now the most powerful man in the country. Yeltsin saved Gorbachev from the hard-liners he himself appointed. Gorbachev is now the junior partner in the relationship. He has lost his power base and is struggling to survive. Gorbachev's role has become that of the principal defender of the union. He and Yeltsin are dependent on each other to move the reform process forward. The defeat of the coup has greatly strengthened the hands of reformers in the Soviet Union. Gorbachev has appointed a new cabinet composed largely of Russian reformers and Yeltsin allies. He has promised new national elections for all posts, including the presidency, immediately after the signing of an all-union treaty, and has supported early action on a new national constitution. The Communist Party has collapsed. The key role played by the party in the coup attempt sealed its fate. Gorbachev has quit as party leader, dissolved its leadership, and said the party should be transformed into a new entity. The powers of the internal security and intelligence agency, the KGB, will be severely restricted. Its role in internal affairs will be substantially altered or eliminated, leaving it in charge of only foreign intelligence. Finally, the defeat of the coup and the removal of the old guard has improved the environment for accelerated economic reform. Gorbachev has appointed a strong advocate of market reform to head a new committee on the economy. But progress on the economic reform agenda is likely to await resolution of the debate over the fate of the union. Reforms cannot proceed in the current state of political turmoil.It is still too early to judge all the implications for the US of the changes taking place. But it's cl ear that a new epoch in US-Soviet relations has begun and the relationship must be reassessed. First, dealing with the Soviet Union will become more complicated. Instead of dealing with one central government, the US will have to work with the 12 Soviet republics and the Baltic states. Second, there is increasing pressure on the US to help the Soviet Union. To date, the US position has been that any large-scale assistance must follow free elections and market reforms. There is doubt whether the Soviets are in a position today to utilize massive Western aid. The need for assistance in other areas - such as humanitarian, food and medical supplies - is more pressing. In the absence of key political decisions on the future of the union, it is unclear who should receive Western aid, whether it can be used effectively, and how it can best support reform. Until these questions are answered, the US should proceed with caution. THIRD, the US must be concerned about control over Soviet nuclear weapons. As the central government has been weakened, the prospect that republics might gain control over nuclear weapons has loomed larger. This would be a destabilizing development. The US must work with our allies, the Soviets, and the international community to ensure that proliferation does not occur. Fourth, US-Soviet cooperation - which has made much progress in recent years - must continue. Key elements of the US-Soviet agenda should be pursued: ratification of recent arms control agreements on strategic and conventional forces; granting of most-favored-nation trading status to the Soviet Union; and cooperation on regional issues. Talks also should be expanded in other areas, such as negotiations for a comprehensive nuclear test ban. Finally, the coup does not change long-term US interests. The success of democratic and market-economic reforms in the Soviet Union is the best long-term guarantee of US security. Events in Moscow and the republics will have an impact well beyond their borders. The potential benefits of an end to the costly cold war are enormous. If the changes being made are successful, the prospects for world peace and stability will be greatly enhanced.