ACCORDING TO an apocryphal but nonetheless useful story, Robert McNamera, shortly after becoming secretary of defense, asked why so many antennas protruded from the roof of the Pentagon. Receiving no satisfactory answer, on the theory that if any were necessary, their users would scream loudly enough to be heard. According to the story, few complained.

This is the approach to the defense budget taken by a group of Cambridge scholars, among them the distinguished cosmologist and writer, Philip Morrison, in The Price of Defense. They made a major project of burrowing, antlike, through every defense-budget account and compiling a layman's catalogue telling where all that $120 billion goes. And, working on the premise that anything that is really necessary will speak for itself, they find we could do without 40 percent of it.

They do this by defending America's military role as to "defend the United States, to support the defense of Western Europe, Japan, and Israel, and to deter war." They further toss out nearly any weapon system that smacks of being part of the offense, on the grounds that the U.S. is a good guy and doesn't plan to start any wars.

Then, so the shock won't be too great, they recommend that the U.S. should phase out, over the next five to 10 years, all its land-based ICBMs, "mothball" 10 of its 13 giant aircraft carriers, get rid of its amphibious assault forces, and keep the Navy to approximately 200 ships. In the Pacific, U.S. forces should quit bases in Asia, Japan, and the Philippines and stay only on Alaska, Guam, and Hawaii. Meanwhile in Europe, one fifth of the U.S. force there, or 10,000 troops, should withdraw. Research and development should stop on the cruise missile, the new MX and Trident ICMB missiles, the B-1 bomber and other programs, since research is merely a "genetic" reason the United States leads the world in the proliferation of new weapons. the money saved can go to health care, the cities, and employing the work force of the defense industries in more humane tasks.

One can sympathize with these goals and indeed, read the book as a useful guide to many of the follies of our current, disorganized defense policy. The book is already having on impact as a guide to congressman seeking to interrogate military witnesses, and in arms-control circles, where the defense budget has always been a bete noire.

But at a more profound level, this interestint, eloquent appeal for disarmament fails. Like other such tracts, it argues on moral grounds rather than addressing the real reasons why we maintain these forces or the practical consequences of dismantling them. For, as the Brookings study points out, military forces do not exist simply to do certain things in war, but to fulfill political and psychological roles in peace.

We keep giant aircraft carriers around because they performed brilliantly in World War II and navies, like elephants, have long memories. Our admirals are far from convinced by the arguments of some arriviste technocrats that new precision weapons make the carrier obsolete. We keep them too, because of widespread perception among U.S. and foreign leaders that naval displays of force are crucial to exerting U.S. influence around the world. We keep our thousand costly land-based ICBM's-and keep making them more able to execute a first strike-not because we will ever use them but because they are the Air Force's principal claim to a role in the hottest defense game in town: strategic nuclear-war fighting. We keep five divisions in Europe not because of their proven value, but because to withdraw even 40,000 U.S. soldiers could wreak havoc with NATO.

These rationales are not necessarily right or morally justified. But they do exist, and in the past they have prevented presidents from making deep cuts in our military forces. Alas, the disarmament movement in this country seems to flourish only at certain historical moments when the general public, not a small group, has become sufficiently repulsed by war, nuclear bombs, burning napalm and the huge amounts of money these things cost to give them up.

We are not in such a phase now. As the forthcoming debate on the new strategic-arms treaty will show, the main battleground of military policy now is between those who would arm us partially and those who would arm us totally. The would-be disarmers are, for the moment, sidelined.

The Brookings study is the more relevant, since it accepts these unpleasant facts of life but asks which forces, in the real world, have been effective and which have not. It examines their efficacy in 215 peacetime incidents, from 1946 through 1975, when U.S. leaders used military force or the threat of it to try to influence political developments abroad.

Based on analysis of 33 of the incidents (such as the 1956 Suez crisis and our intervention in the Dominican Republic in 1961, 1963 and 1964), the authors find that the U.S. uses force most successfully when: (1) it has recently been involved in a war in the region; (2) it has been actively involved in the situation from the beginning; (3) the Soviet Union is not a major player in the immediate crisis, and (4) ground troops or land-based aircraft, rather than purely naval force, are applied. An intriguing finding is that the U.S. tends to use its force when the stock market is high, and presidential popularity is either very high or low.

The authors discuss some of the policymakers' pet assumptions: Does the withdrawal of U.S. troops from an area really destablize it? Has U.S., influence declined in this incidents as the Soviet Union has become more nearly our equal in the ability to wage strategic nuclear war? But they fail to be definitive; as I said, their "findings" are based on analysis of 33 incidents, a sample even the authors admit is not representative of the whole group. The study would have been much more compelling if it analyzed all 215; now, it is merely an intriguing stab at the problem.

But the authors are on the right track: we do need to know, in an era of rising costs, which military forces are effective and which can be scrapped. The answer would be easy if we pulled back to defend ourselves, Western Europe, Japan and Israel. But this isn't likely.