The extent to which a weapon discriminates between military objectives and protected persons and objects depends usually on the manner in which the weapon is employed rather than on the design qualities of the weapon itself. Where a weapon is designed so that it can be used against military objectives, its employment in a different manner, such as against the civilian population, does not make the weapon itself unlawful. 

It is a war crime to employ acts of violence not directed at specific military objectives, to employ a method or means of combat which cannot be directed at a specific military objective, or to employ a means or method of combat the effects of which cannot be limited as required by the law of armed conflict. 

The existing law of armed conflict does not prohibit the use of weapons whose destructive force cannot strictly be confined to the specific military objective. Weapons are not unlawful simply because their use may cause incidental casualties to civilians and destruction of civilian objects. Nevertheless, particular weapons or methods of warfare may be prohibited because of their indiscriminate effects … Indiscriminate weapons are those incapable of being controlled, through design or function, and thus they can not, with any degree of certainty, be directed at military objectives. 

Weapons that are incapable of being controlled enough to direct them against a military objective … are forbidden. A weapon is not unlawful simply because its use may cause incidental or collateral casualties to civilians, as long as those casualties are not foreseeably excessive in light of the expected military advantage. 

Weapons that are incapable of being controlled (i.e., directed at a military target) are forbidden as being indiscriminate in their effect … A weapon is not indiscriminate simply because it may cause incidental or collateral civilian casualties, provided such casualties are not foreseeably excessive in light of the expected military advantage to be gained. 

attacks that employ a method or means of combat that cannot be directed at a specific military objective (e.g., declaring an entire city a single military objective and attacking it by bombardment when there are actually several distinct military objectives throughout the city that could be targeted separately). 

International law also forbids the use of weapons or means of warfare which are “indiscriminate.” A weapon is indiscriminate if it cannot be directed at a military objective or if, under the circumstances, it produces excessive civilian casualties in relation to the concrete and direct military advantage anticipated. 

It is a war crime to employ acts of violence not directed at specific military objectives, to employ a method or means of combat which cannot be directed at a specific military objective, or to employ a means or method of combat the effects of which cannot be limited as required by the law of armed conflict. 

Finally, with the poor track record of compliance with the law of war by some nations, the United States has a responsibility to protect against threats that may inflict serious collateral damage to our own interests and allies. These threats can arise from any nation that does not have the capability or desire to respect the law of war. One example is Iraq’s indiscriminate use of SCUDs during the Iran–Iraq War and the Gulf War. These highly inaccurate theater ballistic missiles can cause extensive collateral damage well out of proportion to military results. 

A weapon must be discriminating, or capable of being controlled (i.e., it can be directed against intended targets). Those weapons which cannot be employed in a manner which distinguishes between lawful combatants and noncombatants violate these principles. Indiscriminate weapons are prohibited by customary international law and treaty law. 

Some weapons, though capable of being directed only at military objectives, may have otherwise uncontrollable effects so as to cause disproportionate civilian injuries or damage. Biological warfare is a universally agreed illustration of such an indiscriminate weapon. Uncontrollable effects, in this context, may include injury to the civilian population of other states as well as injury to an enemy’s civilian population. Uncontrollable refers to effects which escape in time or space from the control of the user as to necessarily create risks to civilian persons or objects excessive in relation to the military advantage anticipated. International law does not require that a weapon’s effects be strictly confined to the military objectives against which it is directed, but it does restrict weapons whose foreseeable effects result in unlawful disproportionate injury to civilians or damage to civilian objects. 

Existing laws of armed conflict do not prohibit the use of weapons whose destructive force cannot be limited to a specific military objective. The use of such weapons is not proscribed when their use is necessarily required against a military target of sufficient importance to outweigh inevitable, but regrettable, incidental casualties to civilians and destruction of civilian objects … I would like to reiterate that it is recognized by all states that they may not lawfully use their weapons against civilian population[s] or civilians as such, but there is no rule of international law that restrains them from using weapons against enemy armed forces or military targets. The correct rule of international law which has applied in the past and continued to apply to the conduct of our military operations in Southeast Asia is that “the loss of life and damage to property must not be out of proportion to the military advantage to be gained”. 

The Law of War does not ban the use of weapons when their effects cannot be strictly confined to the specific military objective. But this rule is true only so long as the rule of proportionality is not violated. However, a weapon which is incapable of being controlled, and thus will cause incidental damage without any reasonable likelihood of gaining a military advantage, is illegal.  

International law also forbids the use of weapons or means of warfare which are “indiscriminate.” A weapon is indiscriminate if it cannot be directed at a military objective or if, under the circumstances, it produces excessive civilian casualties in relation to the concrete and direct military advantage anticipated. 

It is a war crime to employ acts of violence not directed at specific military objectives, to employ a method or means of combat which cannot be directed at a specific military objective, or to employ a means or method of combat the effects of which cannot be limited as required by the law of armed conflict. 

Finally, with the poor track record of compliance with the law of war by some nations, the United States has a responsibility to protect against threats that may inflict serious collateral damage to our own interests and allies. These threats can arise from any nation that does not have the capability or desire to respect the law of war. One example is Iraq’s indiscriminate use of SCUDs during the Iran–Iraq War and the Gulf War. These highly inaccurate theater ballistic missiles can cause extensive collateral damage well out of proportion to military results.