GI Jane, again: the Army tries to sneak women into combat, and some congressmen try to stop it.

LAST year, the U.S. Army redesigned its combat brigades into "units of action": task-organized, self-contained organizations that include support troops--or forward support companies (FSCs)--embedded within them. The change was designed to make our troops more rapidly deployable. Unfortunately, it has been adulterated by a bow to political correctness that threatens to attenuate the military's effectiveness.

Current regulations prohibit women from ground combat--a prohibition that also applies to women in FSCs. Nevertheless, the Army, claiming that there are not enough men to fill positions in the FSCs, has begun to assign women to them. Army leaders claim that this change is consistent with current regulations, and that Defense Department rules bar women from FSCs only during moments in which those units are "conducting" combat. The implication is that women would be withdrawn from them before the onset of hostilities.

House Armed Services Committee chairman Duncan Hunter (R., Calif.) is not amused by this novel interpretation. Accordingly, he asked Rep. John McHugh (R., N.Y.) to introduce an amendment to the defense-authorization bill that bars women from serving in FSCs. This legislative effort is well founded. There are good reasons for women not to be involved in ground-combat support.

To understand these reasons, one must understand that the fundamental nature of war, so well described by Carl von Clausewitz 170 years ago, has not been significantly altered by technological advances. It is still, as Clausewitz wrote, a complex phenomenon, highly influenced by chance and uncertainty.

An important element of war is "friction," which Clausewitz described as "the only concept that more or less corresponds to the factors that distinguish real war from war on paper." Clausewitz used the term "friction" to describe the cumulative effect of the small, often unnoticeable events that are amplified in war, producing unanticipated macro-effects:

   Countless minor incidents--the kind you
   can never really foresee--combine to
   lower the general level of performance, so
   that one always falls far short of the
   intended goal.... The military machine--the
   army and everything related to it--is
   basically very simple and therefore seems
   easy to manage. But we should keep in
   mind that none of its components is of one
   piece: each part is composed of individuals
   ... the least important of whom may
   chance to delay things or somehow make
   them go wrong.... This tremendous friction,
   which cannot, as in mechanics, be
   reduced to a few points, is everywhere
   in contact with chance, and brings about
   effects that cannot be measured, just because
   they are largely due to chance.

The military tries to reduce the natural friction of combat through training, discipline, regulations, orders, and what Clausewitz calls "the iron will of the commander." One particularly important way of countering friction is to promote unit cohesion. Unit cohesion in combat is far more than just "teamwork." Cohesion arises from the bond among disparate individuals who face death and misery together. This bond is akin to what the Greeks called philia--friendship, comradeship, or brotherly love.

There is substantial evidence to suggest that the presence of women in combat would undermine unit cohesion and thereby increase friction. Indeed, friction resulting from the presence of women in the military has already manifested itself in three ways: problems arising from physical differences between men and women; the emergence of double standards that result from these physical differences, undermining fairness and trust; and the replacement of philia by eros. All of these factors undermine morale and may lead to failure on a future battlefield.

As I wrote in these pages in December, a major source of increased friction is traceable to innate differences between the bodies of men and women. A partial catalogue of these differences would include the following facts: The female solider is, on average, about five inches shorter than the male soldier. She has half the upper-body strength, lower aerobic capacity, and 37 percent less muscle mass. She also has a lighter skeleton, which leads to a higher incidence of structural injuries than among men. These differences have had an adverse effect on the U.S. military's effectiveness. They are also excellent reasons to keep women out of FSCs. Women may be able to drive five-ton trucks, but need a man's help if they must change the tires. Women can be assigned to a field-artillery unit, but often can't handle the ammunition. These are exactly the sorts of physically demanding jobs a mechanic in an FSC would be expected to perform.

Second, the physical differences between men and women have generated a series of undeniable double standards in the military. In practice, the desire for equal opportunity usually becomes a demand for equal results, and brings about a watering down of standards in order to accommodate women's generally lower physical capabilities. No one can deny that "gender norming" is widespread in the military. In fact, every service has lower physical standards for women than for men.

Unfortunately, this practice undermines morale by attacking the crux of the military ethos: fairness. Such double standards cause resentment in many military men, a resentment that leads to cynicism about military women in general--even those who have not benefited from a double standard and are performing their duties with distinction.

Finally, one consequence of having men and women live in close proximity under battlefield conditions is to unleash eros on an institution that depends for success on philia. Unlike philia, eros is individual and exclusive. Eros manifests itself as sexual competition, male protectiveness, and favoritism. As former secretary of the Navy James Webb has observed, "[T]here is no greater or more natural bias than that of an individual toward a beloved. And few emotions are more powerful, or more distracting, than those surrounding the pursuit of, competition for, or the breaking off of amorous relationships."

Only a feminist ideologue (or a P.C. military officer) would deny the destructive impact of such relationships on unit cohesion. Does a superior order his or her beloved into harm's way? What happens to morale and unit cohesion when the superior demonstrates favoritism toward the beloved? What about when jealousy rears its head? And what about the possibility that favoritism will be perceived, even when it does not exist?

Social engineering cannot change the fact that men treat women differently than they treat other men. Research shows that romantic relationships among co-workers do not enhance efficiency, and in many cases adversely affect morale and teamwork. But while lives are not at stake in the civilian workplace, they are at stake in military combat.

Of course, there have been incidents in Iraq in which women have performed well in a combat emergency. The mainstream media have tended to fixate on these incidents. ABC News aired a story in March about a woman in a National Guard military-police unit, and the Washington Post had a similar piece after a House subcommittee voted to bar women from FSCs. But such isolated cases don't refute the general argument that widespread assignment of women to combat units would be dangerous because of its impact on unit cohesion and morale.

Apart from questions of the military's effectiveness, there's one last issue to consider: By and large, most women in the Army and the other services don't want to be assigned to combat units or the support units embedded with them. If the Army is worried about not being able to recruit enough women, then putting women on the slippery slope to combat assignments surely isn't the best way to solve the problem.

As Elaine Donnelly of the Center for Military Readiness has remarked, the question of women in combat is not a "women's issue"; it is a national-security issue. That's something we should all be concerned about--and Congress is right to get involved.

Mr. Owens is an associate dean of academics and a professor of national-security affairs at the Naval War College in Newport, R.I.