Armed Conflict Foreign Relations & International Law

The Jus in Bello of White Phosphorus: Getting the Law Correct

Matthew J. Aiesi
Tuesday, November 26, 2019, 8:00 AM

Focusing on the weapon, and not how it is being used, muddies the law and facts surrounding the circumstances in which a war crime may have occurred.

U.S. Air Force Douglas A-1E Skyraider drops a white phosphorus bomb, 1966 (Source: Wikimedia/U.S. Air Force)

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As Turkey’s military incursion into Syria to target Kurdish forces continues to unfold, reports have surfaced that Turkey used white phosphorus munitions during its military operations. The reports inspired calls for investigations and accusations of war crimes. Even members of the U.S. Senate are involved, asking the executive branch if “chemical weapons” were used and if Turkey violated its “international obligations under the Chemical Weapons Convention or any other provision of international law.” Many of the reports on the use of white phosphorus are overly broad, confusing it with either a “chemical weapon” or an “incendiary weapon,” and at least one article suggested it as a “banned” weapon. The use of white phosphorus by Turkey in Syria raises several legal issues that remain poorly understood. However, the consistent controversy regarding the use of white phosphorus in armed conflict and—to a lesser extent—the use of incendiary weapons, has a much broader reach than the current conflict, as the uses of white phosphorus by the United States in Syria and Iraq, by Saudi Arabia in Yemen, and by Israel in Gaza demonstrate.

As the sampling of linked articles illustrates, the reporting on the use of white phosphorus in military engagements wrongfully tends to focus on whether the use of the munition was unlawful. But this misses the distinct legal analysis that is required to determine whether a war crime was committed. It is critical for policymakers, international institutions, the media and legal advisers to understand the jus in bello applicable to the use of white phosphorus. White phosphorus munitions are per se a lawful weapon that can be used against the enemy consistent with the normal laws of targeting. However, white phosphorus munitions, like any lawful weapon, can be used in numerous unlawful manners, such as to specifically target civilians or launch attacks indiscriminately. Focusing on the weapon, and not how it is being used, muddies the law and facts surrounding the circumstances in which a war crime may have occurred. This may ultimately make the actual war crimes harder to identify and prosecute. To that end, this post details the “black-letter” law of white phosphorus, firmly establishing it as a lawful weapon under the law of armed conflict (LOAC), and addresses the common misunderstandings of this weapon under the LOAC.

What Is White Phosphorus?

White phosphorus (WP), also known as tetraphosphorus, is a toxic, colorless, white or yellow waxy solid with a garlic-like odor. It does not occur naturally and is manufactured from phosphate rocks. White phosphorus ignites when it reacts with oxygen, producing thick clouds of white smoke and reaching temperatures high enough to burn through metal. It is used by militaries in various types of munitions primarily but not exclusively for marking or illuminating a target or masking friendly force movement by creating smoke. The restrictions that states place on the use of WP munitions vary along a legal and policy spectrum. I discuss the bases of these restrictions below.

The Law of Armed Conflict Applicable to White Phosphorus

Before diving into an analysis of the applicable jus in bello regarding WP, it is important to highlight that international law is fundamentally permissive for state actors.The Lotus principle, that sovereign states may act in any way they wish so long as they do not contravene an explicit prohibition, applies to the LOAC. Phrased another way, international law is prohibitive law. Determining that it is illegal to use a certain weapon in armed conflict requires a positive showing of its treaty- or customary-based prohibition.

White phosphorus is often incorrectly labeled as an “incendiary weapon” or a “chemical weapon.” Under the LOAC, it is legally neither. The Protocol on Prohibitions or Restrictions on the Use of Incendiary Weapons (Protocol III) of the Convention on Certain Conventional Weapons (CCW) is the treaty regime that governs incendiary weapons, while the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction (CWC) governs chemical weapons. I address each in turn.

White Phosphorus Is Not an Incendiary Weapon Under Protocol III

Two fundamental components of Protocol III are often overlooked, and this has added to the confusion regarding WP and Protocol III. First, Protocol III does not ban the use of incendiary weapons during armed conflict, but it proscribes their use in four specific ways, which are described below. Second, Protocol III’s four specific limitations on the use of incendiary weapons are designed to protect civilians, not combatants.

Article 1 of the protocol defines an “incendiary weapon” as “any weapon or munition which is primarily designed to set fire to objects or to cause burn injury to persons through the action of flame, heat, or combination thereof, produced by a chemical reaction of a substance delivered on the target.” Article 1(b)(i) excludes from the definition munitions “which may have incidental incendiary effects, such as illuminants, tracers, smoke or signalling [sic] systems.”

White phosphorus is primarily designed to take advantage of its smoke-producing properties to mark or illuminate targets, mask friendly force movement, and the like. The incendiary effects of WP are incidental to the illuminant and smoke effects it is designed to produce. Thus, WP munitions fall squarely into the exclusions of Protocol III’s definition of an “incendiary weapon.”

Napalm is the most infamous example of an incendiary weapon that is governed by Protocol III. Napalm is specifically designed to set things on fire. However, while napalm meets the definition of an “incendiary weapon,” its use, like all other Protocol III-governed weapons, is not per se prohibited but is only subject to the four specific and narrow limitations found in Article 2.

Article 2, titled “Protection of civilians and civilian objects,” prohibits four uses of incendiary weapons:

- Making civilian or civilian objects the object of attack by incendiary weapons.

- Attacking a military objective located within a concentration of civilians with air-delivered incendiary weapons.

- Attacking a military objective located within a concentration of civilians with a non-air-delivered incendiary weapon, except when such military objective is clearly separated from the concentration of civilians and all feasible precautions are taken to minimize collateral damage.

- Making forests or other kinds of plant cover the object of attack by incendiary weapons except when such natural elements are used to cover, conceal or camouflage combatants or other military objectives.

Protocol III does not ban or place limits on the direct and deliberate use of incendiary weapons against enemy personal, but it does limit how they can be delivered to the target. The United States is a state party to Protocol III, but Turkey is not. For states that are parties to Protocol III, there is a two-step analysis for determining the jus in bello for attacks with incendiary weapons. First, the munitions in question must be “primarily designed” to set fire to objects or to cause burns to persons, and, if so, then, second, the employment of the incendiary weapon is subject to the four specific limitations for the protection of civilians.

Therefore, because WP is not “primarily designed” to set fires to objects or cause burns to people, it is not strictly governed by Protocol III. Despite the significant secondary incendiary effects, even if Protocol III did govern the use of WP, its direct use against enemy personnel would not be prohibited. This is not a loophole but instead is exactly what states intended under Protocol III.

White Phosphorus Is Not a Chemical Weapon Under the CWC

The CWC bans the use of chemical weapons. It also prohibits the development, production, acquisition, stockpiling, transfer and retention of the same. It is an encompassing treaty; however, not all toxic or harmful chemicals with military applications are prohibited.

The CWC has a two-step definition for what constitutes a “chemical weapon.” First, a chemical weapon is a munition or device “specifically designed to cause death or other harm” by means of a “toxic chemical.” “Toxic chemical” is defined as “[a]ny chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals” (emphasis added). The CWC also has mechanisms for the verification and listing of known toxic chemicals and their precursors, which are incorporated into the CWC through the treaty’s annexes. The CWC does not prohibit these chemicals when they are intended for purposes not proscribed by the treaty.

While WP is fundamentally a chemical agent, and is toxic, it is not governed by the CWC. The CWC prohibits chemical weapons such as nerve agents, asphyxiating gases and blistering agents. These weapons rely on the toxic properties their base agents have when they interact with human physiology—that is, “life processes.” The mere fact that WP produces incredible temperatures by a chemical process does not mean that it is a prohibited chemical weapon. Importantly, tetraphosphorus is not a listed chemical in the CWC annexes. Further, I have found no recorded instance in modern warfare of states deliberately employing WP for its toxic effects.

White Phosphorus Is Not Prohibited by Customary International Law

Since incendiary weapons in general, and WP in particular, are not categorically prohibited by any treaty, the question of their lawful status and use turns on whether incendiary and WP munitions are prohibited by customary law, either as a category of weapons or specifically as anti-personnel weapons.

The weight of state practice is decidedly against the recognition of such a prohibition. As the current case involves Turkey, a NATO member, it is useful to survey state practice from other NATO countries as well as Australia and New Zealand (two states that are historically and routinely involved in combat operations with NATO) to understand what, if any, customary prohibitions on WP exist. It is also worth noting that the International Committee for the Red Cross (ICRC) in its Customary International Humanitarian Law Study (CIHL) has suggested that there is a qualified prohibition against the deployment of incendiary weapons as anti-personnel weapons.

After reviewing the military manuals of the United States (para. 6.14.2), Canada (para. 521.3.), the United Kingdom (para. 6.12.6), France (pages 20-21), Norway (paras. 8.52-8.56), Germany (paras. 453-558), Denmark (paras. 4.2-4.3, and 5.4.2), Australia (paras. 4.30-31) and New Zealand (para. 7.7.7), and the ICRC CIHL Rule 85, I can make two general but instructive conclusions. First, the U.S., Canada, France, Denmark, Germany and Australia have no unique prohibition on using incendiary weapons or WP directly against enemy personnel. Second, Norway, New Zealand and the U.K. essentially treat WP as an incendiary weapon and place qualified limits on its use. These three states, along with the CIHL, limit the use of incendiary munitions to targets such as armored vehicles, bunkers and built-up emplacements only when required by military necessity, and when enemy personnel are not the direct object of attack. The ICRC phrases the limitation as “the anti-personnel use of incendiary weapons is prohibited, unless it is not feasible to use a less harmful weapon to render a person hors de combat (out of the fight).” According to these documents, the prohibition on using incendiary weapons directly as anti-personnel weapons is based on the LOAC principle of unnecessary suffering. I will refer to this as the qualified prohibition rule.

Before turning to the flawed analysis that underlies the purported qualified prohibition rule, it is important to make a few points about the ICRC study. First, as a general issue, the methodology the ICRC used to reach its conclusions has been fairly challenged and cannot be fully relied on as a statement of crystallized customary law for every proposed rule. As it applies to the use of incendiary weapons, the ICRC relied on

just four states (Belgium, Colombia, Sweden and Norway) and the Soviet Union for the assertion “that the use of incendiary weapons against combatants is prohibited because it causes unnecessary suffering.” This is simply insufficient state practice to solidify it as customary law, especially when numerous states do not hold the same opinion. Second, as established previously, WP is not an incendiary weapon, so Rule 85 would not regulate its use. If a state, as a matter of policy, chooses to treat WP as an incendiary weapon, that does not, as a matter of treaty interpretation or custom, mean that WP meets the definition of an incendiary weapon for the rest of the international community.

At first glance, the limitations on using WP or incendiary weapons found in the manuals from the U.K., Norway and New Zealand may appear to support the CIHL’s conclusion that there is a customary qualified prohibition on the anti-personnel use of incendiary weapons, presumably including the deliberate use of WP’s incidental incendiary effects. However, not only is the underlying legal analysis of this position wrong, but the broader approach is fundamentally flawed as well. At best, this formulation creates a new, weapon-specific targeting rule supported by neither the Hague Regulations nor Additional Protocol I (AP I). This qualified prohibition rule unhelpfully blurs the distinct LOAC principles of “necessity” and “unnecessary suffering” together into a new “feasibility” test, applicable only to incendiary weapons. There are no examples of any other conventional weapon burdened by a similar customary rule.

At its worst, the rationale behind this purported qualified prohibition rule endorses the universally derided concept of Kriegsraison, described in 1921 as “[t]he doctrine practically is that if a belligerent deems it necessary for the success of its military operations to violate a rule of international law, the violation is permissible.” This is exactly what Norway, New Zealand the U.K. and the ICRC are endorsing: that the need to attack enemy personnel in a protected site (or otherwise render them hors de combat) is sufficient military necessity to violate the prohibition of using weapons that cause unnecessary suffering.

If a weapon is designed or used with the intent to cause unnecessary suffering, it is illegal to use in all circumstances. If not, then the same analysis could apply to the use of nondetectable fragmentation devices, poisons, booby-traps disguised as civilian objects, and other prohibited means and methods weapons of warfare. As the Hostage Case made clear, “[m]ilitary necessity or expediency do not justify a violation of positive rules.”

The LOAC Principle of Unnecessary Suffering Does Not Prohibit Using Incendiary Weapons or White Phosphorus Directly Against Enemy Personnel

The use of incendiary weapons or WP directly against enemy personnel is not unbridled but is subject to the same rules as all other lawful weapons. Any attack using incendiaries or WP must follow LOAC principles of military necessity, distinction, proportionality and unnecessary suffering.

It is long established in treaty and custom that belligerents are forbidden to use weapons that cause unnecessary suffering, dating back at least to the Saint Petersburg Declaration in 1868, the Hague Regulations of 1899 and 1907, and AP I. This prohibition has been reinforced recently in the International Criminal Tribunal for the former Yugoslavia (ICTY) Statute, the Tadic case and the Rome Treaty. Lastly, in the Nuclear Weapons Advisory Opinion, the International Court of Justice (ICJ) described the prohibition against unnecessary suffering as one of the “cardinal principles contained in the texts constituting the fabric of humanitarian law,” citing to Hague IV, Art. 23.

However, within the landscape of the LOAC, there are two slightly differently worded versions of the rule, which, if applied to the same weapon, could result in different

conclusions regarding its lawfulness. Hague IV, Art. 23, and the ICTY Statute, in pertinent part, state the rule as “it is especially forbidden … [t]o employ arms ... calculated to cause unnecessary suffering…” (emphasis added). Other sources, such as CCW and AP I, state the rule as “the principle that prohibits the employment ... of weapons ... of a nature to cause … unnecessary suffering” (emphasis added). The rule as stated in Hague IV and affirmed by the ICJ is the proper articulation of the rule. This articulation establishes a legally objective and clear standard for the prohibition without inviting a wholly philosophical inquiry as to a weapon’s nature apart from its actual design.

Incendiary weapons and WP munitions are not calculated or designed to cause unnecessary suffering. These weapons fulfill many legitimate and necessary purposes including, as mentioned above, marking targets, illuminating an area, creating smoke to screen troop movements, screening civilians to aid in their escape, and destroying fuel and ammunition caches. If the tactical situation exists, these weapons are also effective when used directly against enemy personnel to induce them to flee or surrender, causing panic to nearby units while blocking the enemy’s line of sight, or any other lawful purpose necessary to defeat the enemy. Any weapon used in war can, and will, cause great suffering to belligerents. War is brutal, and it has limits, but a prohibition against using incendiaries or WP directly against enemy personnel is simply not one of them.

Focusing on the Actual Law of War Violations and Not on the Munition

The foundational rule in the LOAC is that parties to a conflict must at all times distinguish between civilians and combatants, and attacks may be directed only against combatants. Deliberately attacking civilians who are not taking a direct part in hostilities violates the Geneva Conventions, Hague Regulations and customary law. As several reports have indicated, Turkish attacks in Syria against the Kurds have caused numerous civilian casualties, including children. As with incendiary weapons, WP munitions and all other weapons, making civilians or the civilian population the object of attack is strictly prohibited. Moreover, by focusing on the munition and not the established principles of targeting, critics risk allowing the actual violations of the law of war—and potentially the crimes against humanity Turkey may be committing—to be eclipsed by a sentastional yet senseless debate over WP. Every effort should be made to investigate instances of civilian casualties to determine if Turkey has committed violations of international law, regardless of the weapon used to do it. Switching the focus off of the otherwise legal weapon of WP and back onto a proper understanding of the LOAC will enable states to focus on the facts that matter. In turn, this may help states and international bodies develop the proper factual and legal basis to invoke Turkey’s state responsibility for its international wrongful acts and to take appropriate actions against those responsible for war crimes.

Matthew J. Aiesi is a Major in the United States Army. He in as an Associate Professor in the National Security Law Department at The Judge Advocate General's Legal Center and School in Charlottesville, Va. The views expressed here are his personal views and do not necessarily reflect those of the Department of Defense, the United States Army, The Judge Advocate General's Legal Center and School, or any other department or agency of the United States Government.

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