Proportionality in the Law of War

by Maj Gregory G. Gillette

Ever since Warfighting1 was published in 1989, the concept of maneuver warfare has defined and distinguished the way the Marine Corps fights. Maneuver warfare is a series of rapid, violent, and unexpected actions that create a turbulent and rapidly deteriorating situation with which the enemy cannot cope.2 Our ultimate goal in maneuver warfare is panic and paralysis-an enemy who has lost the ability to resist.3 The effective warfighter is “ruthlessly opportunistic, actively seeking out signs of weakness against which [he] will direct all available combat power.”4 Contrary to attrition warfare, maneuver warfare concentrates strength against critical enemy vulnerabilities in order to cause the greatest damage to our enemy’s ability to fight. For example, we are taught to bypass the enemy strength and strike the enemy weakness-employ our infantry company against the enemy platoon vice company against company so that we quickly and decisively defeat the enemy with overwhelming force.

Unfortunately, too many Marines, including a few judge advocates, misunderstand and misapply the law of war (LOW) principle called proportionality, mistakenly thinking that proportionality requires the commander to employ no more force than necessary to defeat the enemy and accomplish his mission. Not only is that idea contrary to maneuver warfare, it’s inconsistent with the LOW. It is legal and tactically sound to squash the proverbial enemy fly using a sledgehammer in order to accomplish the mission.

Understanding Proportionality

Proportionality is one of four basic principles in the LOW. The other three principles are military necessity, discrimination, and unnecessary suffering. To fully understand and properly apply proportionality, we need to understand the other three. The first principle, military necessity, justifies all measures that are indispensable for securing the complete submission of the enemy as soon as possible and that are not forbidden by the law of war.5 In order to be a lawful target, the objective of the attack must offer a military advantage.6

Military advantage can include weakening the enemy-for example, destroying or degrading his troops and assets, creating confusion, limiting his mobility, or limiting his command and control capabilities by degrading his ability to “see” the battlefield. Alternately, military advantage can include strengthening our forces-for example, protecting our troops and assets, creating surprise, or enhancing our intelligence-gathering ability. Military advantage does not include killing civilians or destroying civilian objects.7

Objectives that offer military advantage are termed military objectives. Military objectives are targets that by their nature, location, purpose, or use make an effective contribution to military action and whose total or partial destruction, capture, or neutralization offers a definite military advantage to the attacker in the circumstances ruling at the time.8 People are military objectives if they are enemy combatants or if they directly or actively participate in hostilities.9 Things like tanks and artillery are military objectives because they contribute to military action by their very nature. Places like bridges or roads are military objectives if they provide a military advantage at the time-for example, providing a resupply route to the enemy. Conversely, objects that are purely civilian in nature offer no military advantage and are, therefore, not valid targets.

The second principle, discrimination-sometimes called distinction-requires that military forces distinguish between military objectives and civilians or civilian objects. We know from the above discussion of military objective that commanders cannot target civilians or civilian objects but can only target military objectives.10 In fact, commanders have an obligation to take reasonable precautions to spare civilian life and property during attacks against military objectives whenever possible.11 However, if civilians or civilian objects are used to contribute to the military action, they may lose their status as civilians or civilian objects and become valid military objectives. For example, if a civilian hotel is used to billet and train enemy troops, the civilian hotel becomes a valid military objective because it is now contributing directly to the military action.

The third principle, unnecessary suffering, states that parties to a conflict do not have unlimited right to choose methods of warfare.12 The employment of arms, projectiles, materials, or methods calculated to cause unnecessary suffering to enemy combatants is unlawful.13 This principle recognizes that suffering is an inherent consequence of armed conflict but humanely seeks to minimize suffering where possible. The principle of unnecessary suffering outlaws suffering for the sake of suffering. For example, it’s illegal to put glass inside grenades in order to prevent doctors from locating the glass shards with x-ray machines when they treat wounded combatants.

This principle often rests on the intention of the commander. Lawful weapons can be used unlawfully if the commander selects the weapon specifically to cause unnecessary suffering. For example, white phosphorus rounds are lawful and effective for many purposes. However, if the commander chooses to not use available high-explosive rounds in favor of white phosphorus rounds against enemy troops because he wants to burn, disfigure, and cause the enemy to suffer, he violates the principle of unnecessary suffering.

Finally, we get to proportionality. The principle of proportionality recognizes that some civilian life and property will be destroyed during armed conflict. Proportionality excuses collateral damage to civilian property or incidental civilian death or injury that occurs during an attack on a valid military objective, as long as the collateral damage or incidental civilian death is not excessive in relation to the concrete and direct military advantage anticipated from the attack.14 Thus, proportionality begins with identifying the valid military objective and identifying any collateral damage or incidental loss of life foreseeable from the attack. The commander then weighs the foreseeable collateral damage or incidental loss of life against the expected military advantage to determine whether the collateral damage or incidental loss of life is excessive. As long as the collateral damage and incidental loss of life is not excessive compared to the military advantage, then the attack does not violate the principle of proportionality.15 If there is no collateral damage, then proportionality has no effect on the size or type of weapons used against the enemy.

Assume, for example, that your unit locates a squad of Iraqis in the desert. Also assume that there are no civilians or civilian property anywhere in the area that could be harmed from attacking the Iraqi squad. There will be no proportionality issue present regardless of the size or combination of weapons delivered because there will be no civilian death or damage to civilian property. The commander is free to select and employ any weapon or combination of weapons he chooses, and he cannot violate the principle of proportionality. The commander may violate the principle of unnecessary suffering or other rules in the LOW, but not the principle of proportionality.

Why, Then, Is Proportionality Misunderstood by So Many?

Proportionality is often confused with a similar sounding LOW concept called “proportionate response.” Unlike proportionality, proportionate response does concern the amount of force employed against the enemy. Proportionate response, however, is a principle relevant to self-defense, not mission accomplishment.16 It’s important to recognize that the LOW governing self-defense is different than the LOW governing offensive use of force to accomplish the mission. Fortunately, this is one area where the law makes practical sense.

The LOW recognizes that the commander’s objective in mission accomplishment is to offensively seize, break, destroy, or kill the objective. If we apply maneuver warfare we want to use overwhelming force to achieve our mission to physically defeat the enemy and destroy him psychologically. The good news is that, other than the principles discussed above that are designed to protect civilians and minimize unnecessary suffering of combatants, the LOW really doesn’t care whether we kill the enemy with a teeny, tiny bomb or use a really big bomb. Therefore, it’s legal, and consistent with maneuver warfare, to smash the proverbial enemy fly with a sledgehammer.

In self-defense, by contrast, the objective is to protect ourselves or protect others against a threatened or actual use of force. Accordingly, the rules for the use of force during self-defense require that we use the minimum amount of force necessary to adequately defend ourselves-a measure of force proportionate to the threat. Common rules of engagement (ROE) in self-defense state that:

Force used in self-defense to counter a hostile act or demonstrated hostile intent must be reasonable in intensity, duration, and magnitude to the perceived or demonstrated threat.

Properly drafted ROE recognize the difference between the principles of proportionality during targeting for mission accomplishment versus the principle of proportionate response during self-defense. Properly drafted ROE recognize that while force used in self-defense must be reasonable and proportionate in response to the hostile threat or act, force used in mission accomplishment can be overwhelming and unreasonable. Unfortunately, many ROE intermix and confuse these terms using the term proportionality when discussing self-defense or using the idea of proportionate response when discussing mission accomplishment.17

Why Should We Care?

Discussing the differences between proportionality during mission accomplishment and proportionate response in self-defense is not merely an academic debate. The real danger is that ROE that confuse these two principles unnecessarily limit the amount of force a commander can use to accomplish the mission. The confused ROE erroneously prevent the commander from decisively engaging the enemy with overwhelming force and unnecessarily restrict the commander to weapons and amounts of force that are “proportionate” to his enemy target.

Let’s revisit the Iraqi squad in the desert armed only with small arms. ROE that confuse these principles might erroneously state that a commander can only use proportionate amounts of force against the enemy to achieve his mission. The commander acting under these ROE would be limited to engaging the Iraqi squad only with small arms weapons. Therefore, instead of quickly destroying the Iraqi squad using overwhelming force, the commander gets bogged down in a proportionate firefight with the enemy unnecessarily risking time and men.

Although not acceptable, the confusion is understandable because the confusion starts at the very top. The Chairman of the Joint Chiefs of Staff Standing Rules of Engagement (CJCS SROE) confuse these terms. For example, the CJCS SROE use the term proportionality to discuss both selfdefense and mission accomplishment. More recently, the ROE for Operation ENDURING FREEDOM (OEF) erase all difference between these two concepts. The OEF ROE state:

If use of force is necessary for mission accomplishment OR to respond to a threat or provocation, that use of force will be proportional in that it should be reasonable in intensity, duration, and magnitude. . . .18

Thus, documents like the CJCS SROE and the OEF ROE teach our Marines that proportionality and proportionate response are one and the same.

Another danger is that non-governmental agencies (NGOs) and international scholars increasingly confuse the principle of proportionality in mission accomplishment with the idea of proportionate response in self-defense. The confusion is sometimes innocent but other times intentional. These scholars incorrectly argue that international law requires warring nations to achieve military objectives using proportionate amounts of force. For example, after the first Gulf War, some NGOs and international scholars accused the United States of violating international law, erroneously claiming that we violated the principle of proportionality by using disproportionate force to accomplish the mission.

Some of these scholars honestly confused these two principles and ignorantly accused the United States of violating the principle of proportionality. Other scholars purposely confused these principles in an attempt to make the fight “fair” and reduce the United States’ ability to use overwhelming force upon the enemy. I expect that we’ll see the same allegations about the war in Iraq. Accordingly, Marines at all levels need to understand the principle of proportionality in order to effectively defend against these groundless allegations in the future.

The Fix

The only way to end the confusion is to first recognize the differences between these two concepts and secondly to strictly adhere to the established vocabulary to identify these two concepts. In the LOW arena, proportionality should only refer to balancing military objective against collateral damage during mission accomplishment. Likewise, the term proportionate response should only refer to the amount of force used during self-defense to respond to the threat. By using these terms precisely we can further the understanding of LOW and ensure that our commanders aren’t unnecessarily limited when they fight to accomplish the mission. By understanding and using these terms correctly we add the sledgehammer back to the commander’s arsenal when he fights the proverbial enemy fly.


  1. Marine Corps Doctrinal Publication 1, (MCDP 1), Warfighting.
  2. Fleet Marine Force Manual 6, Ground Combat Operations, Chapter 1.
  3. MCDP 1, p. 74.
  4. Ibid., p. 75.
  5. Field Manual 27-10, The Law of Land Warfare, Department of the Army, Paragraph 3a.
  6. Hague Regulation No. IV, Annex to the Convention: Regulations Respecting the Laws and Customs of War on Land, 18 October 1907, Article 23.
  7. Protocol additional to the Geneva Conventions of 12 August 1949, and relating to the protection of victims of international armed conflict (Protocol I), 8 June 1977, Article 51(1-3).
  8. Ibid., Article 52(2).
  9. Ibid., Article 51(3); Article 3 common to the four Geneva Conventions.
  10. Ibid., Article 48.
  11. Ibid., Articles 57 and 58.
  12. Ibid., Article 35(1).
  13. Ibid., Article 35(2).
  14. Ibid., Article 51(5)(b).
  15. Ibid.
  16.  Proportionate response is also relevant to the concept of reprisals. However, reprisals are seldom used today.
  17. There are times when the political leadership wants to limit the amount or type of force used during mission accomplishment in order to promote the strategic political goals.
  18. CJCS, Washington, DC, 061130Z October 2001 ROE Authorization Serial One, OEF.