AS THE SENATE opens debate on a bill to finance registration of 19- and 20-year-old men for a possible military draft, it's useful to keep in mind that the "draft debate" is not what it once was. In the several years since troubles in the all-volunteer force started being aired, and especially in the nearly six months since Jimmy Carter proposed registration, a discussion has been conducted of the full range of the nation's military manpower needs. The debate is no longer just about the draft, which is the particular manpower issue of the most immediate and volatile public impact. It is also about recruiting and retention, pay and benefits, increments of preparedness and "signals" of readiness, the nature of the forces the United States may require in future contingencies and the effect of military service on the relationship between individual citizens on the one hand and the society and the state on the other. It has been one of our better national debates.

As a result, certain things now be said with a measure of assurance. The first is that registration -- and one cannot talk about registration without anticipating an eventual draft -- is a limited but useful tool for a nation attempting to play a responsible leadership role in an uncertain world. There is a lot that draft won't do.It won't help retain skilled NCOs, for instance, or top off combat readiness or, by itself, rebuild the reserves. For these objectives other measures are required. But the draft will contribute to raising the quality (and equalizing the social burden) of those who serve, priming the reserves and preparing for unexpected larger manpower needs. As part of a comprehensive approach, it's sensible.

Something else important has been learned, too: the rest of the world is watching. The United States is one of the few developed nations to rely exclusively on volunteers to fill its armed forces. Even those (few) countries that understand why the United States abandoned the draft regard the volunteer system as a strange gamble on fair weather in stormy climes. Typically, West Germany's chancellor says, "There's a difference between a country that has a military service obligation . . . and a country which has abolished the draft." Reinstating registration has, at this point, little to do with President Carter's post-Afghanistan intent to send Moscow a message of American resolve. It has a lot to do with informing other countries that the United States is serious for the long haul. Now that the issue has been raised, a refusal to move to registration would throw a long shadow. A Senate debate with a positive answer -- the House has already done its work -- would give the best results, both at home and abroad.