THIS IS HOW the U.N. conference on disarmament, opening today in New York, came to be: The "non-aligned" nations, unified on little else, started demanding a world conference on disarmament 20-odd years ago. The United States and China, separtely fearing Soviet exploitation of theissue, resisted, and no conference was held. By the mid-1970s, however, as William Sweet has pointed out, Soviet arms programs and arms transfers has grown to a scale denying Moscow political advantage from a disarmament conference, and Washington and Peking eased their objections. The non-aligned movement had meanwhile switched its principal focus in reorganizing the world economy, and many of its members were themselves scrambling for arms. The point is that although by now no one was really red-hot for it, the thing went forward, and the U.N. Special Session on Disarmament is the result.

On at least two levels, the session has useful work to so. First, it can try to describe the problem accurately, avoiding the hyperbole and hypocrisy that have tainted the ideal of disarmament over the years. Why does not remember, for instance, the Soviet Union's repeated appeals at the United Stations for "general and complete disarmament"? - a cynical travesty on the hopes of people everywhere for peace. True, fantastic sums - $400 billion a year worldwide by one count - are spent on defense; and it is tempting to comtemplate what could be done if those resources were diverted to civilian needs. Yet though the "arms race" does waste resources and retard economic and social growth and nourish aggressive anbitions, it also provides security and self-confidence and allows aggrieved parties to right wrongs and in some respects helps maintain peace. It does no harm to talk straight about these things.

On second level, the delegates can talk shop. There may be no "new proposals" in disarmament, or in arms control, the term we prefer. But there are degrees of progress. Some states may offer statements of unilateral self-denial. That's fine. But the more common and reliable procedure is for pairs of groups of rival states to work out explicit or parallel restraints. Here is where the United States comes in. Vice President Nomdale is evidently to give a status report on American arms-control negotiations with the Soviet Union. Some Americans and some foreigners feel it is bad for the disarmament cause, and for the Americans image, that President Carter himself won't be in New York. But the cause and the American image - and the Carter image - do not need speechifying. They need real progress in the negotiations under way. The special session shouldbe judged by what impetus - if any - it gives to achieving those restraints on arms development and acquistion that are within nations' reach.