ONE OF THE BIG STORIES of 1981 is the defense budget. National security is back in fashion, and our government will spend about $200 billion on military programs, primarily to counter the Soviet Union. Such programs are necessary, but they do not exhaust the question of security. In addition to threats from relations among nations, there are threats arising from the relationship of humanity to nature. "Regrettably," says Lester Brown in Building a Sustainable Society, "non-military threats to a nation's security are much less clearly defined than military ones." Yet to focus only on the military threats is like defending the front gate of a fortress while leaving the rear entrance unattended.

Brown identifies three nonmilitary threats. One is soil erosion. Although world food output has doubled since 1950, the inherent productivity of one third of U.S. cropland is now falling because of the excessive loss of topsoil. Current commercial agricultural practices are mining our soil at the very time that the world is becoming increasingly dependent upon the North American breadbasket. Yet we have virtually no idle cropland, and world grain reserves are only 40 days of world consumption (compared with 102 days 20 years ago).

A second threat is the unsustainable pressure that our civilization is putting on three natural biological systems: forests, grasslands, and oceanic fisheries. Overfishing, overgrazing, and deforestation have become widespread. The third threat is the potential depletion of oil reserves before alternative energy sources are devloped. In Brown's view, "We live in a petroleum culture that is fast running out of petroleum."

Growing world population and rising consumption help to feed these threats. Ecological stresses and resource scarcities translate into economic stresses, and "ultimately, they translate into social stresses--hunger, demoralization, forced migration, higher infant mortality, and reduced life expectancy." Our attitude toward nature must change if a "sustainable society" is to evolve. "If we abandon our exploitative relationship with nature, we may be less inclined to exploit each other. At the international level, we may begin to see that the real threat to the long-term security of nations and of civilization itself lies less in military conflict than in the unsustainability of society as it is currently organized."

Brown's book can be read on several levels. It is a fascinating, although sometimes inconclusive, collection of facts and quotations on a wide scope of subjects ranging from windmills in Sweden to birth control in China, and crop rotation in Iowa. It is a tocsin warning us against the key ecological and resource threats. And it is yet another in the tradition of books expressing the "green philosophy" of ecologism. The reader can profit from one of the levels without necessarily accepting all three.

As with many works in this genre, there is a strange absence of hard political realities in Brown's book. One can agree on the need for a broader view of security and yet wonder how it relates to the traditional view. This is not discussed. Brown laments that governments of developing countries find it easier to justify expenditures for the latest models of jet fighters than for family planning. But military security is often a real need--witness Cambodia, Chad or Afghanistan. When Brown writes that "the Mayans, the Mesopotamiams, the Carthaginians--these culturally extinct people who lived in an era when environmental tensions mounted slowly--probably did not understand the forces undermining their economies," one cannot but think that the Roman military threat came first for Carthage.

Of the three nonmilitary threats, Brown's energy projections are the most questionable. He claims that oil is one of the easiest components of the future energy budget to project, with output declining 5 percent by 1990 and an additional 10 percent by 2000. But the 2 trillion barrels of ultimately recoverable reserves that he cites will last well past the turn of the century at recent annual world production rates of about 22 billion barrels. Brown acknowledges that politics rather than absolute scarcity will affect production rates in the next two decades, but he has little to say about the politics of oil. We are told that "OPEC is one of the world's most influential organizations," and "a godsend," but little about the role of Saudi Arabia, the real power behind OPEC, or the international politics that have so drastically affected production rates in the Persian Gulf three times in the past eight years.

On the other hand, Brown projects renewable energy sources as exceeding petroleum in world energy consumption in the year 2000. He suggests that solar energy's share may be substantially higher than the 20 percent goal for the United States announced by President Jimmy Carter in 1978 and regarded by many energy experts at that time as a "political number."

The social philosophy of ecologism expressed in the book has many attractive values such as simplicity, frugality, equity and harmony with nature. But the political and economic causal relations are not adequately developed. The relationship of equity, sustainability and growth can be the source of serious conflicts which are not explored. Sometimes the change that Brown says is needed appears to be radical; at other points it seems that current market and regulatory mechanisms will suffice. At one point we are told not to count on technology for solutions. At other points (for example, solar energy) technology comes to the rescue. We are told not to count on the market mechanism to fill its historical role of quickly substituting one commodity for another because interdependent factors are approaching planetary limits together. Yet there is little evidence produced for the proposition. And assertions that shifts in the price relationship of oil and grain "may signal the beginning of a long-term transformation in the relative prices" of renewable and nonrenewable resources are not compelling when one realizes that other commodities such as copper have moved in the opposite direction.

Judged as a book of social philosophy, the verdict must be that Brown's case is "not proven," but the specific warnings in his book are important (even though debatable in the case of oil). Moreover, the general message that we cannot afford to ignore the long-term and nonmilitary dimensions of security in this year of the defense budget is surely right.