Sometimes statistics point straight to what is in people's minds. Such a statistic is this: 400,000 of the 9 million young Americans required by a 1980 law to register for the draft have not done so. Consider how improbable that would have seemed 20 years ago, before Vietnam, in an America as close in time to D-Day as we are today to the Gulf of Tonkin resolution. Then, it was taken for granted that society had a legitimate right to conscript young men and that the armed forces had a legitimate use for their services. Today, apparently, these things are no longer taken for granted -- at least by large numbers of the group of Americans most directly affected, young men age 18 to 21.

One reason is that our highest officials themselves have been hesitant in defining and enforcing society's interest. The draft registration law was passed at the urging of President Carter, as one response (the now defunct grain embargo was another) to the Soviet invasion of Afghanistan. Candidate Ronald Reagan in 1980 attacked the law as immoral, and President Reagan in 1981 said he would not enforce it. Only this year did the administration change its mind and announce that it would prosecute violators, beginning, appropriately, with those who proclaimed their defiance. In the circumstances, one can understand why many young men might conclude that the registration requirement was anything but urgent--a vast change from 20 years ago when it was unheard of for a young man not to register.

Another reason for the change in attitudes is, of course, Vietnam. One of the most affecting things about last week's Vietnam veterans' march was the fact that it was one of the first celebrations of those who heeded their government's call and served. For many Americans, the heroes of Vietnam are those who refused to serve, those who protested rather than those who complied. Many young men born between 1960 and 1964--those who are supposed to register now -- must have drawn the lesson from recent history that it is honorable not to obey but to protest.

They may draw another lesson: that without the existence of large conscript armies you will not have wars. This is not the only lesson that one can draw from history, but each generation inevitably draws lessons mostly from what it has seen firsthand. If Mr. Reagan and others, who remember how lack of vigilance can also lead to war, want young men to understand that, they are going to have to take the trouble to educate them more than they have.

In the short run, that means continuing prosecutions under the draft registration law, not only of those who have proclaimed their violations, but also, as the administration is beginning to do, going after the many others who have not. The draft registration law cannot achieve its full purpose -- demonstrating resolve to potential aggressors--if it is not enforced and if hundreds of thousands continue to ignore it. The decision by a Los Angeles judge Monday that the law is invalid and that the government has used invalid selective prosecution to enforce it should be appealed and will likely be speedily overturned. The law is clearly constitutional, and the First Amendment protects the right to protest, but not the right to violate, a valid law.

Beyond that, our leaders have a more important duty. If society is to assert a claim to young men's services -- even through as unintrusive a means as a requirement to register -- it needs to make plain to them why that claim is legitimate.