The American Civil War saw the first use of many weapons that would revolutionize warfare—armored warships, submarines, land mines, and machine guns to list a few. One Civil War innovation would have a more dubious fate, however, becoming the first weapon specifically outlawed by international treaty. This was a rifle bullet that would explode on impact with a human body. Immediately after the war, the U.S. Army Ordnance Department concluded that exploding rifle bullets were inhumane, and should never be purchased again.

Experience during the Civil War had shown that any soldier hit by an ordinary rifle bullet was likely to be put out of action by that wound alone. In most cases, exploding bullets merely made an already disabling wound worse. The suffering caused by such bullets was therefore militarily unnecessary and inhumane.

The same conclusion had been reached at almost the same time by the Russian government, which had also developed an exploding rifle bullet. Unlike the United States, however, Russia faced a number of potentially hostile land powers, and the czar’s government was reluctant to give up any new weapon unless it could be assured that potential enemies would also forgo its use. In 1868, therefore, the Russian government convened an international conference at St. Petersburg to consider this issue. (The United States did not participate—this was an era of isolationism in U.S. foreign policy.)

The resulting treaty, called the St. Petersburg Declaration, banned the use of exploding or incendiary bullets weighing less than four hundred grams. It also stated certain important general principles, as follows: “That the only legitimate object… during war is to weaken the military forces of the enemy; that for this purpose it is sufficient to disable the greatest possible number of men; that this object would be exceeded by the employment of arms which uselessly aggravate the sufferings of disabled men, or render their death inevitable; that the employment of such arms would, therefore, be contrary to the laws of humanity.” These principles are now accepted as part of customary international law, binding on all nations, whether or not they are parties to the St. Petersburg Declaration.

International conferences meeting at The Hague in the Netherlands in 1899 and 1907 distilled the St. Petersburg principles into the familiar principle that it is forbidden to use weapons that cause “unnecessary suffering” or “superfluous injury.” This principle, which is also part of customary international law, was reaffirmed in the first Additional Protocol of 1977.

This rule prohibits, for example, the use of explosive projectiles filled with clear glass. Glass fragments would make a soldier’s wounds more difficult to treat, because a surgeon would have trouble seeing them. For similar reasons, it is forbidden to use explosive projectiles designed to injure with fragments not detectable by X-rays. The rule against the use of dumdum bullets designed to flatten on impact also reflects the principle against unnecessary suffering and superfluous injury.

In applying the principle against unnecessary suffering and superfluous injury, the military advantages of the weapon must always be weighed against the suffering it causes. After all, the very phrase “unnecessary suffering” implies that there is such a thing as necessary suffering. A weapon cannot be considered forbidden simply because, in the abstract, it produces great suffering; the military side of the equation must always be considered as well.

It should be noted that a weapon may be unlawful if used for one purpose, and yet lawful if used for another purpose. For example, during World War I, the British armed the machine guns on their warplanes with incendiary bullets. The initial reaction of the German government was to threaten to try captured British airmen as war criminals for violating the St. Petersburg Declaration. Upon reflection, however, the German government backed down. Today, legal experts generally regard the use of incendiary and small caliber explosive bullets as lawful in air warfare, though their use in infantry rifles would still be forbidden.

For the same reason, the use in land warfare of shells containing white phosphorus—which burns on contact with air and can cause exceptionally severe burns—is permitted to provide a smokescreen, for marking targets or against enemy vehicles.

Customary international law also prohibits the use of indiscriminate weapons. An indiscriminate weapon is one that cannot be directed at a legitimate military objective. The V-2 rockets used by Germany in World War II were indiscriminate weapons, in that they could not be directed at any target smaller than an entire city. After the 1991 Gulf War, the U.S. Department of Defense reported to Congress that the SCUD missiles used by Iraq (which were not very much more accurate than the V-2) were indiscriminate, and that their use constituted a war crime.

One weapon that has provoked particular controversy in recent years is the cluster bomb. A “cluster bomb unit” (CBU) consists of a dispenser containing several hundred small bombs (or “submunitions”). After being dropped from a warplane, the dispenser opens at a preset altitude to disperse the submunitions over a wide area. The use of CBUs by the Israeli air force in Lebanon against the PLO in the 1970s and again against Hezbollah in 2006 was widely criticized by human rights organizations.

Cluster bombs are often denounced as “indiscriminate” weapons, but this charge is not, strictly speaking, accurate. CBUs can be directed very effectively against area targets such as tank formations, military bases, airfields, rail yards and similar legitimate targets and are not therefore indiscriminate as that term is used in international law. As with any other weapon, cluster bomb should not be used in situations where expected civilian casualties are likely to be excessive in relation to the military advantages expected from the attack.

Even when cluster bombs have been used legally, unexploded submunitions can be dangerous. Hundreds of submunitions will be dispersed in a typical cluster bomb attack (for example, each U.S. CBU-87 carries 202 submunitions). Even if 95% of the submunitions explode as designed, ten or more undetonated “dud” bomblets will be left on the field from each CBU dropped. These can create danger for civilians, peacekeeping forces and other noncombatants for decades after the conflict ends.

International law has only begun to deal with the dangers posed by the explosive remnants of war, including unexploded CBU bomblets. In November 2003, negotiations were concluded on a new treaty, Protocol V to the United Nations Convention on Certain Conventional Weapons, that would obligate countries at war to record where unexploded ordnance could exist (e.g., where CBUs have been used) and to make this information known at the end of hostilities. Another approach being pursued is to develop cluster bombs whose unexploded submunitions would self-destruct or deactivate after a given period.

(See biological weapons; chemical weapons; mines; poisonous weapons.)