What are US vital interests?

WHEN Secretary of Defense Caspar W. Weinberger told the National Press Club that the United States military force should be used only in defense of US ''vital interests'' and then only to ''win,'' he was expressing the consensus of virtually the entire uniformed US military establishment, active and reserve.

More than that, in his condemnation of a ''gradualist'' response to ambiguous international crises he was stating a political fact of life that no United States statesman or political party can afford to ignore. The political fact of life is that one major political party - the Democrats - has very nearly been destroyed, at least in terms of viability at the national level, by its ''gradualist'' management of the Korean war, the Vietnam war, and the Iranian hostage rescue mission.

Critics of Mr. Weinberger's speech - including indirectly Secretary of State George P. Shultz - are saying that the world is too complex to preclude continued ''gradualist'' involvement in a crisis, and that the US government may not be able to wait for the sort of national and congressional consensus Weinberger says is necessary to risk the lives of American servicemen and women.

If political science existed in a vacuum, Weinberger's critics might be right. But in terms of politics as practiced in the US, Weinberger's viewpoint can be ignored only at the peril of yet another disaster. In short, anyone who has lived through the past 35 years should know that Americans will fight again only when they can be persuaded that there is reason to fight, and that once they decide to fight they are going to finish off whoever drove them to that exigency. Having stood in the streets of Tokyo in 1946 and looked clear to Yokohama across an urban desert, I can only hope that potential foes will take to heart what Weinberger is telling them.

If Weinberger's speech is to be faulted it should be for his imprecise use of the term ''vital interests.''

The government has no standard definition of vital interest. The State Department, which should have promulgated a definition long ago, simply throws out the term to justify whatever policy happens to be current at the moment. Thus, it justifiably became an object of scorn and ridicule during the later Vietnam years.

The Joint Chiefs of Staff developed a definition, but hid it away in their murky, notoriously unread staff documents. Essentially it is this: ''That which pertains to the survival of the US and its free institutions.'' That is, ''vital'' in the most literal and easily understood meaning of the term. In geopolitical terms there are only three areas that can meet such a test: the US itself, Japan, and Western Europe.

Are there, then, no other interests for which the US would risk American lives? Obviously there are, and these the Joint Chiefs have categorized as ''major'' interests. They have never been spelled out to the people whose sons and daughters must do the fighting and dying, because of the military penchant for locking up everything it writes and its dread of open discussion and debate.

The public that pays the bill and that must ultimately pay the price in lives for the calculations or miscal-culations of government has a right to know just what Weinberger is talking about when he uses the term ''vital interests,'' and it has a right to know just where in the catalog of US interests from ''vital'' to ''minor'' any particular foreign involvement stands.

That might impose a more rigorous intellectual burden on our Defense and State Department officials than they are accustomed to dealing with, but there is no reason to believe that it is beyond their capacity. They would find the burden eased considerably if they would release to the public the documents in which these evaluations are set forth and invite the participation of the public in reaching the sort of consensus Weinberger says is so essential.