IT seems extraordinary to speak in casual tones of the death of the Soviet Union, a political entity that dictated a huge proportion of the West's time, energy, and money over the past 40 years. Yet that is what the Dec. 8 meeting between Russian leader Boris Yeltsin, Ukrainian leader Leonid Kravchuk, and Stanislav Shushkevich of Byelorussia represented.The failed August coup accelerated the end of the Soviet empire. But the Dec. 1 vote by the Ukraine to leave the Union was the final blow. Now through these three Slavic republics echoes the word "sodruzhestzvo or commonwealth. It is too early to fathom the finer details of this new proposal. The economic structures of a new commonwealth, the status of nuclear weapons, the governing arrangements, and even the participants are still to be worked out. The republics of Georgia, Moldavia, Azerbaijan, and the Baltics have said they won't join. That leaves Kazakhstan, the southern Muslim republics, and Armenia in question. Given the frightful chaos in and among the Soviet republics, the idea of sodruzhestzvo offers a bit of direction and sanity. The notion could shift the downward political momentum among old republics in a new and constructive direction. Mr. Gorbachev's demand for a central union created ill will and contentiousness among republics whose energies were spent bitterly negotiating a departure from the center. Now, with the idea of the Soviet center gone, these republics will have to spend time considering what they can bring to a common union and how to govern it effectively. They may realize it is in their interest to join forces and build something for the future; the choice of Minsk as a capital indicates a desire to move toward the Western world of commerce and culture. The short-term future won't be rosy. Yet Gorbachev's cries about a Yugoslav-style civil war are overdone. Nightmare scenarios can't be ignored. But unlike Yugoslavia, the ethnic minorities in the Ukraine and Byelorussia (as in the Baltics) voted overwhelmingly to secede from the Soviet center. They want this. Rumors of a military coup abound. A successful coup seems less likely than before. Besides, it may shortly be that republics provide needed military housing and pay, not the center.