Name the weapon in the U.S. arsenal that was not used in Gulf War II because an international treaty prohibited it.

A nuclear bomb? There’s no treaty banning its use. Nerve gas? We destroyed our arsenal years ago. Weapons that tear and burn flesh, kill indiscriminately, and destroy buildings as well as life? No, we used these “conventional bombs” in large numbers; they are legal. The correct answer: the illegal weapon wastear gas.

This chemical weapon (does that make it a weapon of mass destruction?) can legally be used for crowd control by police, but not for war fighting by the military. The international Chemical Weapons Convention-ratified by the U.S. Senate in April 1997-outlawed the blistering aerosols known as mustard gas, the nerve agents sarin and VX, and the biological toxin ricin. But lumped in with these truly nasty-and lethal-chemicals are the mucus irritants CS and CN (chlorobenzylidenemalononitrile and chloroacetophenone), commonly called tear gas.

Why include in this treaty a substance frequently used by police within our own borders-a weapon widely seen as a humane alternative to bullets, and one that would have been useful during the war in clearing mosques and public buildings of the Saddam Fedayeen? Our negotiators struggled to have the tear gas prohibition removed from the chemical weapons treaty, but other countries would not agree. The United States used tear gas extensively in the Vietnam War to force the enemy out of tunnels; many of the Viet Cong were killed, and other countries accused the U.S. of waging chemical warfare. In the end, we compromised, and agreed to the treaty’s tear gas prohibition.

There are no truly 100 percent nonlethal weapons. Last October, Russia used fentanyl, a supposedly nonlethal opiate, to subdue Chechnyan terrorists who were holding hostages in a Moscow theater. The aerosol killed 129 of the roughly 800 hostages. Nobody knows whether a better solution, such as a negotiated surrender, could have been found.

Tear gas, too, has caused unintended fatalities. Nevertheless, it is part of a growing movement in the defense community toward the use of kinder, gentler weapons that can achieve victory without death or destruction. Research on such weapons is accelerating. In 1997, the military created a “Joint Nonlethal Weapons Directorate” to recommend, develop, and field nonlethal weapons. The National Academy of Sciences has issued a series of reports on the potential of these weapons. Ideas include slime that makes the enemy slip, dowel guns that crunch bones but don’t penetrate the body, carbon fibers that short-circuit electric power facilities, skunk-like aerosols that make areas smell bad, impassable goo and foam that denies access to buildings and clogs weapons, and biological agents (more weapons of mass destruction?) that rapidly degrade fuel, metal, and other materials.

You might assume that most people see nonlethal weapons as a good thing, but the tear gas example contradicts that. This is the weapons paradox. If you invent a way to make war less cruel, less destructive, less horrible, be prepared to be branded by some arms control advocates as an evil monster, since you have lowered the threshold by making weapons use more palatable and thus war more likely.

That lesson was learned by our kindest, gentlest presidentJimmy Carter. In the 1970s, our military wrestled with the difficult challenge of how best to protect West Germany. This narrow country, pressed close against the Soviet bloc, was so vulnerable to a surprise attack that the United States would not rule out first use of nuclear weapons in its defense. Nukes were our only credible threat.

But were they truly credible? Would we use them in West Germany? Wouldn’t they destroy the country that we were trying to save? President Carter announced a solution called the “neutron bomb”. And he was pilloried for it.

The neutron bomb was a technological marvel-and to understand it requires a brief lesson in the anatomy of nuclear explosives.

An ordinary hydrogen bomb consists of several parts. The first stage is a fission device called the “primary,” usually a plutonium shell that is imploded to form a critical mass. The gamma rays from this primary move with the speed of light and compress the “secondary,” a combination of lithium-6 (which rapidly decomposes to tritium and helium) and deuterium. The tritium and deuterium (both isotopes of hydrogen) are squeezed and ignited by the gammas to release the energy of fusion. Neutrons from the fusion stream out and are captured on a depleted uranium shell (U-238), triggering additional fission. About half of the energy of such a weapon comes from this secondary fission. The fission fragments from this uranium shell are also responsible for most of the radioactive debris.

The neutron bomb eliminated the uranium shell, and made the primary really small. As a result, most of the energy was released in fast and deadly neutrons rather than in heat and blast. This was a bomb that would kill but not destroy-perfect for the defense of West Germany.

Many arms control advocates viciously attacked President Carter for supporting such an outrageous weapon. By reducing the collateral damage, he was making nuclear weapons more acceptable, and therefore more likely to be used. Carter wasn’t the first to run into this problem. In the early 1960s, the United States had deployed a small tactical nuclear weapon called the Davy Crockett that could be carried and launched by a single soldier. It reduced the nuclear threshold near to zero, and was vigorously opposed by arms control advocates.

Carter found himself in a weird trap. If a new weapon is more destructive than its predecessors, it is clearly evil, on the simple premise that if causing death and destruction is bad, then causing more death and destruction is worse. On the other hand, a less destructive weapon would be more likely to be used, and is therefore also evil. Thus, any change in weaponry is evil. Remember this ironic contradiction as you listen to the upcoming debates on nonlethal weapons.

I don’t know the solution to the weapons paradox. From 1961 to 1971, the Davy Crockett was deployed in Europe; did that make the region more stable or less stable? In 1999, China announced it had developed a neutron bomb. Did that increase the danger of armed conflict with Taiwan and the United States or decrease it? Is the ban on tear gas justified by the benefits of the Chemical Weapons Convention? The answers are not obvious, and there are no simplistic principles that obviate the need for detailed analysis.

I wonder what would have happened if the neutron bomb had come first. Suppose President Carter had then proposed replacing it with a bomb that not only killed people, but also destroyed cities and left a deadly radioactive legacy, making reconstruction impossible for decades. Would he have been hailed as a savior, for replacing the neutron bomb with one that made nuclear war less likely?