THERE ARE nearly 30 million soldiers in the world, and the United Nations estimates that more than 200,000 of them are children. Many are fighting and dying every day, even when the headlines fail to remind us. The "1990 Yearbook" of the Stockholm International Peace Research Institute is sobering reading. It documents 32 major armed conflicts during 1989 alone, with the brutal new war in Liberia too new to make the list. In all, more than 20 million people, most of them civilians, have died in armed conflicts around the world since 1945, with no sign of real change.

Total disarmament may be too much to ask for. Self-interest and fear may prevent it forever. Persuasive people will argue the need for aggression or defense or deterrence, and enough people will be persuaded to cause trouble. Remember, too, that war is one of the biggest, most successful businesses in the world, to the tune of nearly $900 billion in expenditures worldwide in 1990 alone.

But how about a day, just a single day of peace? Could we not at least try that? No monarchs would fall, no one's borders would be threatened, no one would lose a sale or a job -- not in just a day. January 1 of the year 2000 marks the beginning of a new year (calendar purists will argue for 2001), a new decade, a new century and a new millennium. Most of the people alive today, and the vast majority of all the children alive today, will witness this extraordinary day.

Why not begin the next thousand years with a day of peace on earth?

For much of the world, the day will be a holiday anyway. Even if we did no special planning, fighting would slack off. Why not make the moratorium complete?

It will take a great deal of work by many political, religious and military leaders, many governments, many service organizations and many private citizens around the world to engineer a global truce, but it's well within the realm of the sober realities under which we live.

How do we convince all of the relevant parties to lay down their arms, and how can we prevent some trigger-happy bully from ruining the day? Should we pay people off who cannot otherwise be persuaded? Should we (could we) trade wheat for cease-fires? A decade of planning and hard work will yield reasonable answers to such questions. We have a decade. We can do it in a decade. It's a reachable goal.

The behavioral sciences tell us that a little goal-setting can go a long way. If we work together to create this remarkable day, we will behave better toward each other along the way. We will long for this day and envision this day, and we will be better for it.

If we achieve this day of peace, it will be remembered for all time by all humanity. It will tell us that the cycle of war can be broken, that peace is truly within our grasp, that humanity, with all its flaws and in all its diversity, is good.

And maybe -- just maybe -- this day without war, this single day of perfect peace, will last another day.

Robert Epstein is the founder and director emeritus of the Cambridge Center for Behavioral Studies in Cambridge, Massachusetts.