WHEN EAST Germany begins using the West Germany Deutschemark as its currency today, the unification of Germany will become an accomplished fact. Many important issues will remain to be settled among the Germans and the four allies of World War II. Legally and formally, unification still lies at some point in the future -- perhaps the end of this year. But in most of the aspects that touch people in their daily lives, Germany is now one country.

People can walk freely back and forth across the border through the gaps in fences and walls that until last fall were among the most heavily guarded in Europe. Western goods are pouring into East Germany, their bright packaging crowding the shelves of socialism's gray and dowdy stores. Western businesses from banks to fast-food chains are rapidly opening branches in East German towns. The West German parties are hard at work organizing among East German voters in preparation for the election that they hope will be held in December -- the first free all-German election in nearly two generations.

It's a time for celebration. These enormous changes mean far better lives for many millions of people. While much attention is being paid to the financial costs of unification, they are not crucial. West Germany, with its enormous trade surpluses, has the resources to rebuild the eastern economy. There will be a ripple of higher interest rates and inflationary pressures, but they will be manageable.

The deeper question is German power and the uses to which it is put as it grows. In statistical terms, unification does not greatly increase it. United Germany's population will be a fourth larger than the present West Germany's, and its economy bigger by perhaps a sixth. The real difference has less to do with size than perspective. Central Europe is now reemerging, looking both east and west.

At a great price in danger and suffering, the Cold War and the division of Europe imposed a remarkable stability. Now that the Cold War has faded, it's understandable that there are fears for the stability that it produced. But unlike the old Central Europe, the new one is firmly committed to democracy and a market economy. It's the West German constitution as well as the West German D-mark that the East Germans are embracing. The forces that plunged Europe into a spiral of violence and evil in the first half of this century are not present today. It's quite right to be attentive to Europe's stability, and it's necessary to expect that in the 1990s it won't be quite what it was in, say, the 1970s. But when you consider conditions in Eastern Europe in the 1970s, you will probably agree that a little movement is not a bad thing.