IN THE WEEKS since American women soldiers fought alongside men in Panama, a consensus has begun to emerge that women should be allowed to serve in combat units if they want to. A recent New York Times/CBS News poll says that seven out of 10 Americans now hold this view.

I respect and admire women who want to serve, but I believe an expanded role for women in the U.S. armed forces is a demonstrably bad idea.

Mine is an unusual perspective. I am a Vietnam veteran, a social scientist and a journalist who has done some reporting on the military. In 1967 and 1968 I was a junior officer in a Seabee battalion that provided construction support to the 3rd Marine Division near the demilitarized zone separating the two Vietnams. Until 1984, I was an anthropology professor who tried to teach his students to understand why cultures develop the way they do, and that history is not just the story of one damn thing after another.

The simple truth is this: However well one woman (or 1,000 women) may have performed in a firefight in Panama, it does not change the fact that men -- as a group -- fight better than women. This fact is as unremarkable (and as unsexist) as saying that young men usually make better soldiers than do men who are no longer young. All other things being equal, an army whose average age is 26 will beat an army whose average age is 46. All other things being equal, an army of men will beat an army of women. All other things being equal, a society that puts women in the field at the expense of fielding a like number of men will lose its wars.

Of course, all other things are never equal. This is why the United States can probably get away for a long time with its policy of using an increasing number of women volunteers to make up for the men who fail to enlist or for Congress's failure to conscript them. Diminishing East-West tensions make a major war seem less likely. Defense cuts, including cuts in personnel, appear inevitable. Since our forces will be smaller anyway, why not let motivated women do as much of the job as they can handle? Isn't it unfair to exclude military women from the most career-enhancing assignments?

Different things are at stake in the short and long terms. I taught my anthropology students that taboos (such as the incest taboo) evolve when the benefits of a contemplated action are immediate and obvious but the costs are veiled and postponed. For untold millennia, every society of which I am aware has had a taboo against sending women to fight while able-bodied men were still available. Now we are on the verge of violating this taboo. Before we embrace the violation as our national policy, we should try to consider the consequences.

The short-term benefits are clear. Americans prize autonomy of the individual. That's what we fought for in past wars, isn't it? Women now make up almost 11 percent of the armed forces. Why exclude them from combat only to replace them with men less eager? The United States may be entering an era of brushfire wars and relatively small regional conflicts that present no immediate threat to the survival of the nation. America is still a vast, rich country that will continue to hold a huge advantage in resources, population and technology over any probable combination of enemies. Under these circumstances an expanded military role for women could cause serious problems, perhaps even defeat, but would not result in the conquest of the United States by foreign powers.

The long-term costs are hidden but deadly. One way to think about these costs is to consider the "law of the minimum," propounded by Justus von Liebig, a 19th-century German chemist and a pioneer in agricultural research. He discovered that plant growth is limited if one necessary factor is unavailable -- even if all the other factors are available in abundance. Applied to human societies, Liebig's "law of the minimum" suggests that survival is not geared to coping with good conditions, or even to average conditions, but to an ability to get through the worst crises.

Militarily, that worst crisis is total war, which has come to the world twice in this century. Both times America got off easy in the expenditures of one necessary factor -- manpower.

Unlike Europe, the United States never fully mobilized its population for military service. Even during World War II, we didn't push up against Liebig's law of the minimum. On Sept. 20, 1943, Gen. George C. Marshall testified before the Senate against a bill that would have deferred from the draft men who were fathers before Pearl Harbor. Similar bills would have been laughed out of the legislatures of the European powers. Take a look at photographs of soldiers in the European armies of both world wars. You will see faces of many older men, many of them fathers. Still, they did not draft women. And with the exception of the Soviet Union, which ran out of men in World War II, they didn't push women toward combat. There are two good reasons for this: Men generally fight better than women, and men generally fight better when women aren't around.

A misunderstanding of the Israeli example clouds the issue. Contrary to popular belief, Israel does not conscript women to fight -- and certainly not so that able-bodied men can be excused from military service -- but to free men to fight in combat units. Israelis I've talked to think that the U.S. trend of pushing women toward combat to take the place of men who stay home is folly.

It is often said that modern technology has erased the physical advantage men have brought to war. This is fantasy. It is not even true in support units. I recall several occasions in Vietnam when I grew faint after manhandling (good word, that) ammunition crates or sandbags for hours in the blinding heat. The point is not that some women could have done better; it is that most women could not have done as well.

It is also said that sexual distraction in military life is an issue only for relics like me, and that today's more enlightened generation of young men develop nothing but brotherly affection for their female "buddies." Not only does this go against all experience and common sense, but I found it to be false when reporting on U.S. forces deployed to the mountains of Honduras in 1988. While frustration, heartbreak and jealously did not seem to be problems for the Army reservists and National Guard members who came into the camp and returned to the United States after a few weeks, they certainly were present among the sexually mixed camp cadre, who had to live with each other for almost a year. Human nature doesn't change, and we are asking for trouble by pretending it has.

It was drilled into my head when I was on active duty that the mission came first and the welfare of the people I led came second. Aren't those who demand equal opportunity for women in combat violating that most basic principle of military leadership? What's good for individual careers isn't necessarily good for the country. The mission of the armed forces is to win wars, not under the best conditions or average conditions but with a margin for error under worse conditions than can be imagined. In a crisis, the country that puts women in the field at the expense of men will lose. Meeting such a crisis successfully is never easy, and it may become impossible if our culture changes to the point where American men are no longer embarrassed to have women do their fighting for them.

During just a few months in 1914, the small, professional British army, which had sustained the world's largest empire for a century, was all but swept away by the tides of continental war. The same thing could happen to us, if not in one year or 10, then in 100. Under those conditions, the issue will no longer be who has the right to fight, but who can be compelled to fight. The British responded by sending pretty girls into the streets, where they would pass out a white feather -- a symbol of cowardice -- to any young man they saw who was not in uniform. In his book "The Myth of the Monstrous Male," John Gordon recounts the story of a society lady meeting an Edwardian dandy during World War I and demanding to know why he wasn't with the boys fighting in France. "Madame," he is reported to have replied, "I am the civilization which they are dying to defend."

What will happen to us when men like that are not the outrageous and memorable exception but the accepted norm, and half of those dying to defend them are women?

Lou Marano is a Washington Post copy editor.