I recently was invited to the Pentagon to watch a film depicting field tests of a new weapons program called the Active Denial System, which, it occurred to me, could have been named by an unhinged cognitive therapist. The live-action video opened on a vista reminiscent of Iraq or Afghanistan. Scattered amid the scrub of a desert plain, angry demonstrators howl unintelligible slogans and advance menacingly on a handful of soldiers who nervously pivot their rifles back and forth trying to deter the mob. For safety's sake during this test run, the ''crowd'' -- played for the most part by off-duty soldiers -- flings bright green tennis balls at the uniformed servicemen instead of rocks. As one member of the crowd hurls a ball, a soldier operating the Active Denial System (it looks like a squat satellite dish) targets an unruly protester in the weapon's viewfinder, squeezes a trigger that releases a beam of energy and, in a split second, one ''civilian'' howls and scampers away, fanning his rear end. Other demonstrators suffer similar fates, yelping and fleeing in panic, as if they have encountered a wall of invisible fire. After tumbling backward, the horde spins around, pointing and hollering like a Stone Age tribe encountering modern weaponry for the first time.

What they were feeling was a blast of electromagnetic energy that causes a great deal of pain but does no lasting harm. That, in essence, is the point of a new generation of nonlethal weapons being developed by the military: to enforce and do battle without killing, or in the words of the Defense Department, ''to incapacitate personnel or materiel, while minimizing fatalities, permanent injury to personnel and undesired damage to property and the environment.'' Along with the Active Denial System, the military is testing bullets that disintegrate in mid-air, propelling their nonlethal payload to their targets, slimy goo that stops people in their tracks and, eventually, guns that shoot pulses of plasma energy that stun and disorient.

In an era when the American military increasingly finds itself in situations where civilians and combatants can be difficult to distinguish between, and when the line between soldiering and policy has blurred, nonlethal weapons could prove useful. At the same time, such nonlethals might be abused, like any other weapon. Still, in a world where the tolerance for ''collateral'' casualties is fast diminishing and where soldiers return home haunted by their ''kills,'' such novel weapons, if made to work, could well make war less hellish. Sue Payton, a deputy undersecretary of defense who screened the film for me, put it this way: ''The less killing we do, the better.''

Imagine the plight of a soldier guarding a crucial road or a checkpoint in a war zone who sees a truck barreling toward him. The driver fails to heed his calls to stop. Does the soldier let the truck keep coming and risk a possible suicide bombing? Or does he shoot and, as more than one American soldier in Iraq has belatedly realized, kill an innocent driver or even an entire family who bore no ill will but simply didn't understand his warnings?

It's a predicament that could one day be solved by a nonlethal weapon, I learned recently from David Karcher, who runs the Joint Non-Lethal Weapons Directorate in Quantico, Va., the military's central research facility for this new generation of armaments. To illustrate his point, Karcher first showed me a film clip of an enormous truck driving over a blanket-size swatch of spiked netting, only to screech to a halt as the wheels and axle get caught in the net. Marines used it successfully in Haiti this spring, Karcher said, and a related technology -- webs of fabric shot out of a cannon into the path of oncoming motorboats -- can entangle propellers and keep suspicious small craft from coming too close to warships. Next, Karcher showed me a video of a liquid weapon so farfetched it could have come from James Bond's arsenal: the Mobility Denial System is a fluid that can be dispensed from a backpack or a tank. ''It's like a thick goo,'' Karcher explained. ''It has the friction coefficient of wet ice.'' Any area sprayed with it instantly becomes impassable. ''You can't walk on it, drive on it; you can't land or take off an airplane.'' Karcher said he had tried it out himself: ''I resembled the scarecrow in 'The Wizard of Oz.' You really can't stand on it.''

Any of these so-called countermateriel weapons might be used to secure a perimeter, deny access to a building or simply make an airfield or another strategic site unusable. But the bulk of the directorate's efforts are focused on what happens when the enemy -- or someone whose intent you can't discern -- manages to get within firing range. ''What we really want to do,'' Karcher explained, ''is give people more choices between shouting and shooting, more tools between the bullhorn and the bullet.''

The Modular Crowd Control Munition -- a device that hurls hundreds of tiny rubber ''sting balls'' -- slows or stops a mob with no casualties. Other weapons currently being employed rely on blunt trauma to deter: the M1006 Sponge Round, a projectile with a foam-rubber nose, can deliver a serious body blow. It originated in the arsenals of law enforcement and was subjected to a battery of safety tests by the military before being used overseas.

Such weapons have their drawbacks, of course. Sting balls, sponge rounds and other projectiles in the nonlethal arsenal have a limited range and accuracy. If a shot is off -- if the victim is hit in the eye, for instance -- he or she may well suffer permanent damage. Moreover, a healthy 20-year-old will withstand a nonlethal assault far better than an 80-year-old who may accidentally find himself caught in the crossfire. But both will end up with nasty bruises.

As the Pentagon works to make these nonlethal systems safer, it also confronts the challenge of how to keep them effective. Susan LeVine, a civilian scientist at the directorate, says that she faces this conundrum daily: ''How can you have a weapon with a nonlethal effect that's good enough to be effective but won't cause serious injury or death?''

The newest generation of nonlethals, now in the initial stages of development, may provide an answer, LeVine said. ''We're about to turn the corner from the blunt-impact munitions to more high-tech, advanced directed-energy capabilities,'' she told me. Asked what ''directed energy'' means, she explained, ''speed-of-light delivery, unlimited magazine, precision effects. You've got beams of energy and minimal collateral damage.'' These proposed new weapons range from programs that use electromagnetic waves or lasers to fry a vehicle's wiring to the Pulsed Energy Projectile, or P.E.P., an embryonic design that one day may fire packets of plasma energy that pummel and disorient people with explosive bursts of light.

The Active Denial System belongs to this class of directed-energy weapons, but it works in a more subtle fashion than the P.E.P., relying instead on millimeter wave energy to heat the top layer of the skin. ''It causes palpable pain,'' LeVine said, ''and the effect is very universal, all ages and genders.'' And unlike many nonlethal weapons, the A.D.S. can operate beyond small-arms range, enabling an operator to deter a foe long before a potentially fatal clash occurs.

Raytheon, which developed the Active Denial System for the Pentagon, says it is testing it in the field and fixing technical glitches before delivering a working system mounted on a Humvee. After a demonstration for the news media scheduled for later this year, the military will decide if it wants to invest additional financing for its development.

LeVine asked if I would like to try it out. She took me over to a small metal box that looked like an air purifier, explaining that it fires a beam of energy like the one used in the A.D.S. I would be on the target end of things. I put my index finger near an aperture on the side of the box. I took a deep breath and moved my finger in front of the hole. ''Nyaaah!'' I cried out. A burning sensation similar to what you would feel touching a hot stove made me immediately withdraw my hand. Almost instantly, the pain faded. LeVine smiled to reassure me and said, ''We've all touched it, and our fingers are still here.''

Nonlethal weapons first gained attention in the 90's, thanks to the efforts of Gen. Anthony Zinni, who was director of operations for American troops in Somalia in 1992. ''When we arrived,'' Zinni told me, ''we were confronted with demonstrations, looting and crowd situations that didn't require the use of lethal force. The troops felt frustrated because they didn't have anything but their rifles and bayonets to deal with the situation. One day I came across some of our troops trying to hook up wires to their car batteries so that they could keep people at bay using electric shocks.'' Zinni quickly banned these makeshift gadgets, but he asked Central Command for nonlethal weapons. All he received were cases of pepper spray.

A year after the United States turned Somalia over to the United Nations, Zinni returned to escort the blue-helmeted peacekeepers as they left the country. This time, he assembled an array of nonlethals that already had proved effective in law enforcement, including ''sticky foam,'' a sprayable substance that can glue a suspect to the ground; stinger grenades that explode into rubber shrapnel that deters; spikes called caltrops capable of puncturing tires; and many others.

Word spread quickly of Zinni's experiments with sticky foam. ''I became the poster boy for this in the beginning, and some senators and congressmen became interested,'' he said. In 1996, Congress created the Joint Non-Lethal Weapons Program, later endowing it with an annual budget of approximately $25 million. The Marines were granted executive responsibility for the program, but each of the services plays a role in development.

The Joint Non-Lethal Weapons Directorate brings its most promising technologies to the attention of the various armed services. In the event a prototype developed by the directorate catches the eye of the Navy, say, that branch assumes responsibility for procuring the system. Law-enforcement officials also monitor prototypes in the hope of finding weapons that can be used in policing.

After a relatively modest beginning, the directorate has won advocates in high places, among them Adm. Arthur Cebrowski, who runs the Defense Department's Office of Force Transformation, established by Donald Rumsfeld to help re-engineer the armed forces into what he envisions as a nimble, mobile, high-tech fighting machine. When I visited Cebrowski recently, he outlined what he calls ''issues of regret'' -- things the military should embrace now or forever rue the consequences. Nonlethals topped the list.

''The way we currently outfit or train our people, they are confronted with these binary choices,'' he said, the most consequential choice being shoot or don't shoot. ''Yet we know that combat doesn't necessarily resolve to binary choices. It's an enormously complex and dangerous undertaking. Shouldn't we have a more nuanced weapons capability to go with this?'' Introducing nonlethals into combat, he contended, ''will change the character of war.'' When I asked him about the bottom-line benefits of this change, his response was blunt: ''The general rule is fewer dead people is better than more dead people.'' He added that he believes there is a ''moral imperative to suppress the violence of statecraft.''

That may seem like an obvious point, but it's a relatively new one for the military to embrace. Mike McBride, a specialist in nonlethal weaponry at Jane's Information Group, an internationally respected firm that gathers and provides military analysis, told me, ''the idea that you can neutralize the enemy without killing them is an increasingly attractive proposition.'' He said that ''we're heading toward the day when, like 'Star Trek,' you can set the phaser on stun. That's the holy grail of less-than-lethal weapons.'' But, he cautioned, ''Whether we'll ever get there, I don't know.''

War is not likely to get any less bloody or deadly anytime soon, supporters of nonlethal weaponry say, unless more money starts flowing through the pipeline. After an initial increase in financing from $9.3 million in 1997 to $43.4 million in 2004, the directorate's budget is slated to increase moderately to $61.3 million by 2009. The Council on Foreign Relations, which has issued three reports on nonlethal weapons over the past decade, recently criticized the current rate of financing and recommended the directorate get between $200 million and $400 million a year.

''The budget of the parking lots at the Pentagon is bigger than the nonlethal weapons program,'' said Janet Morris, president and C.E.O. of M2 Technologies. She and her husband, Christopher Morris, founded M2, a defense contracting and consulting company, to bring promising technologies to the attention of the government. Both have worked as members of the Council on Foreign Relations' task force. Not surprisingly, they are supporters of nonlethal weapons and outspoken critics of the military establishment, which they feel is the principal obstacle to their widespread use. ''The resistance comes from the mind-set of the 20th-century military, which holds that you bring lethal force to bear in an overwhelming way,'' Christopher Morris said.

His comment points to what may be at the crux of the debate over nonlethal weapons: should this country be sending its armed forces into ambiguous situations like Afghanistan, Iraq and Kosovo, where soldiers are expected to perform humanitarian duties in tandem with their traditional roles? Those in the military who see their exclusive role as defending the nation with deadly force say no -- not only to this kind of peacekeeping mission but also to the nonlethal weaponry that could make it more effective.

Indeed, if Zinni's support of nonlethals grew out of the peacekeeping mission in Somalia, so did resistance to them. At the time, an Army officer wrote an article for the Navy publication Proceedings titled ''What Price Sticky Foam?'' claiming that the introduction of nonlethal weaponry would confuse soldiers and erode their fighting edge. Conservative commentators like Jeanne Kirkpatrick, the former United Nations ambassador, weighed in on the subject, claiming that President Clinton had foisted the weapons on Zinni, a claim Zinni dismisses.

Soldiers themselves seem to think nonlethals have their place. Those I spoke to who have used them in combat situations became avid proponents. Lt. Sandy Bucher, a platoon leader stationed in Al Kut, Iraq, served in the first military police battalion in that country to receive nonlethal weapons and ordered the use of sting-ball rounds to confront hostile crowds. Nonlethals, she told me, offer a way out of delicate situations that leaves few, if any, casualties.

It's a refrain I heard repeatedly from soldiers who have used nonlethals. Maj. Steve Simpson, who served in Afghanistan, recently used blunt-impact munitions to protect the perimeter of a compound housing some senior military leaders. The problem of the military, he said, is that ordinarily a soldier has few options between doing nothing and doing everything. ''Nonlethals bridge that gap,'' he said. Simpson, who wrote the first instruction manual on the use of nonlethals in 1996, said that the senior leadership of an Army unit arriving in Afghanistan approached him recently, wanting to know more about nonlethals. ''I was pleasantly surprised,'' he said. ''I see nonlethals as an integral part of all future operations.''

Mindful of the potential of nonlethals to limit casualties and head off public-relations disasters, the military has equipped a growing number of units overseas with Non-Lethal Weapons Capability Sets, a collection of countermateriel and counterpersonnel devices. It's a step toward a more systemic embrace of nonlethals, but a preliminary one at best. Indeed, the tools available in these kits are no more advanced or effective than the ones that accompanied the military in Somalia nearly a decade ago.

A futuristic rifle called the XM29 may one day make it possible for soldiers to toggle back and forth between lethal and nonlethal capabilities in an instant. The gun has two magazines: one in the front containing regular rounds and one in the rear holding a smaller number of very large bullets carrying nonlethal payloads. (A soldier chooses which to use, depending on the situation.) A laser range finder sits atop the whole contraption. ''The same lethal weapon has the ability to use a nonlethal device without changing any of the operating procedures or mechanisms on the weapon,'' explained Col. Peter Janker, who runs the Armaments Engineering and Technology Center at Picatinny Arsenal in New Jersey.

At first glance, the XM29 looked pretty lethal to me, particularly the sizable bullets in back. But the girth of those bullets allow them to carry rubber balls, pepper spray or anything that can be crammed into the projectile. ''The carrier system -- the bullet -- is lethal, no matter what the filling,'' Steve Toth, a civilian contractor who works closely with Janker, conceded. The solution is a curious mix of high-tech and low-tech. When a soldier aims the XM29 and presses the button, a laser range finder measures the distance to the target, and a microprocessor sets a carefully calibrated fuse in the bullet proportionate to the distance between the gun and the target. When the soldier pulls the XM29's trigger, the bullet fires conventionally and then explodes a few feet short of the target. The charge hurls the nonlethal payload forward at a relatively slow speed while simultaneously counteracting the velocity of the bullet.

Of course, there's always the danger not only that the mechanism could misfire but also that someone could step into the flight path after the gunner fires. So scientists are now trying to develop a prototype that will use a ''proximity fuse,'' meaning the bullet will self-destruct as soon as it senses something -- or someone -- in its way.

The directorate is also developing an artillery shell that could turn a conventional mortar into a nonlethal weapon. Like a bullet fired by the XM29, this armament had a similar problem when first conceived. ''You can pack rose petals in a conventional mortar, but the mortar cartridge itself is a brick falling out of the sky,'' Toth said. ''So the trick here is how we slow down the carrier vehicle to a safe speed.'' To show the way it works, Toth picked up a shell casing with what looked like helicopter blades sticking out of it. Using the same aerodynamic principles that enable a maple seed to waft to the ground, the shell can be fired by a conventional mortar and float harmlessly to the earth as it dispenses a nonlethal payload.

But these devices have yet to reach the battlefield, and won't in the near future. For the time being, the only weapon in the field that possesses both a lethal and a nonlethal capability is a standard-issue M4 rifle: one variation includes an M203 grenade launcher with a payload of rubber shrapnel, the other an X26E Taser stun gun attached to the front end, which zaps its victims with an electric current, paralyzing them. ''Now when a person goes into a room, instead of going in with the Taser and having to drop it and transition to the rifle, he can go in with lethal'' (Janker gestured with his trigger finger on the M4) ''or nonlethal'' (he waggled his finger on the Taser). He went back and forth several more times: lethal, nonlethal, lethal.

Janker took the gun with him on a recent trip to Iraq, where it found admirers among the troops. Thanks to Janker, M4-Taser combinations found their way into the hands of soldiers in Iraq. ''Now we'll get some feedback,'' he said.

There's something oxymoronic about ''nonlethal weapons.'' The term invites unrealistic expectations about their safety and application. Indeed, many early proponents of nonlethals wanted to call them ''less-than-lethal weapons,'' in the recognition that mistakes do happen. But the unease nonlethals arouse goes beyond semantics. Stephen Goose, director of the arms division of Human Rights Watch, is critical of what he terms the ''excessive secrecy'' surrounding many nonlethals, particularly exotic technologies like the Active Denial System. ''There hasn't been any policy discussion in public, or legal discussion in public, as to whether this is consistent with international law,'' he said.

Nonlethal armaments do undergo a legal review within the military to ensure compliance with international laws and treaties. Joseph Rutigliano, an attorney with the Marine Corps who is responsible for evaluating the different nonlethal armaments, said that most ''raise very few issues.'' The chief exceptions are those that fall under the scope of the Chemical Weapons Convention, which, Rutigliano noted, ''prohibits the use of riot-control agents such as tear gas in offensive military operations.''

A category of nonlethal armament that might come under scrutiny in this regard is the one known as ''malodorants'' -- weapons that emit highly nauseating smells that can incapacitate. One such agent synthesized under the guidance of the directorate combines the odors of human excrement and rotting flesh. Kansas State University was conducting environmental tests of malodorants as late as last month, but the military claims that, at present, it is not moving forward with plans to ''weaponize'' the odors, partly for fear of violating the Chemical Weapons Convention.

Only a handful of advocates of nonlethals believe the convention should be rewritten to make room for these agents. Janet Morris of M2 Technologies would like to see ''calmative agents'' -- weaponized versions of Valium and other drugs -- deployed in battle. ''The current convention forbids us from using chemical agents to chase people off the battlefield,'' she noted dryly. ''We can't tranquilize them. No, we have to shoot them.''

Some critics of nonlethals argue that even with strict adherence to international treaties, they are still cause for concern. Stephen Goose worries that they will erode the so-called force threshold. While a soldier in Iraq might refrain from using lethal force in certain situations, Goose pointed out, he or she might be tempted to apply nonlethal methods in situations that don't merit it.

Claudio Cordone, the legal director for Amnesty International, is more optimistic. He agrees with Goose up to a point, but he encourages ''anything that provides for a use of force that isn't lethal.'' More broadly, he argues that nonlethals could ''encourage an approach to fighting that minimizes the harm to people and things, even if that harm is lawful under the laws of war. If nonlethals allow for the containment of the savagery of war, it's a good thing.''

Still, if weapons like the Active Denial System leave no mark on a victim's body, couldn't they be used for torture? ''There's always that potential,'' Cordone concedes. And Goose adds, ''What happens when some of these weapons get into the hands of militaries with poor human rights records?'' He paints an Orwellian picture in which repressive regimes obtain nonlethal weapons to keep restive populations in check without resorting to the sort of bloodshed that can earn a country unwanted attention.

It's too early to tell whether directed-energy nonlethals like the A.D.S. might one day be misused or how the next generation of armaments in development will change soldiering. Sue Payton, who showed me the A.D.S. video at the Pentagon, compared the A.D.S. to Wilbur and Orville Wright's first plane. Back then, Payton mused, ''You would have said: 'What can this thing really do? It can only fly 120 feet and can hardly carry a person.''' She paused. ''We're at that stage with directed-energy nonlethals. It's the first baby step.''