Are Thermobaric Weapons Lawful?

The Russian Federation has deployed and likely used thermobaric weapons during its invasion of Ukraine. Ukraine’s ambassador to the United States alleged that Russia “used the vacuum bomb today, which is actually prohibited by the Geneva Conventions,” while American lawmakers and others have cited the use of thermobaric weapons when advocating for direct NATO intervention in the conflict. The White House Press Secretary, responding to “reports of illegal cluster bombs and vacuum bombs being used by the Russians” against civilians commented that “it would potentially be a war crime.”

What are thermobaric weapons? Are they “prohibited by the Geneva Conventions,” or otherwise regulated by international law? This post will apply the law of armed conflict (LOAC) to assess the most common criticisms leveled against these munitions, evaluating them as both means and methods of warfare. While the weapons themselves are not unlawful, their use to directly target civilians would violate the LOAC.

What Are Thermobaric Weapons?

Thermobaric is a portmanteau of the Greek words for heat and pressure. It refers to weapons that inflict damage by maximizing the shockwave and overpressure associated with an explosion. Overpressure involves waves of energy from an explosion that “produce casualties and damage through crushing, bending, tumbling, and breaking,” as opposed to propelling pieces of shrapnel as their primary mechanism of injury.

Thermobaric weapons are commonly known as fuel-air explosives, vacuum bombs, volumetric weapons, dual-stage explosives, or enhanced blast weapons.[i] These weapons are typically filled with aerosolized solid fuel or a highly combustible slurry. Once they reach their target an initiating explosion or “scatter charge” spreads a cloud of the fuel across the target, while milliseconds later a secondary combustion causes that cloud of fuel and atmospheric oxygen to detonate. The result is a massive fireball that produces an especially powerful blast wave, to include negative pressure (the vaunted vacuum effect) as the atmospheric oxygen is consumed by the explosion. This extraordinary pressure collapses buildings, ruptures organs, and negates many forms of cover including underground facilities. Thermobaric weapons have the potential to generate causalities that overwhelm military medical systems, to say nothing of their potential to inflict civilian harm.

Thermobaric weapons are renowned “as much for their psychological effect as for their destructive power.” Russia, China, the United Kingdom, and the United States are all known to have thermobaric weapons in their arsenals, ranging from multi-launch rocket systems to grenades lobbed by individual soldiers.

Modern thermobaric weapons evolved from the BLU-82 “daisy cutter.” These munitions were developed to blast helicopter landing zones out of the Vietnamese jungle, but were later employed against troop concentrations, supply depots, and for mine clearance. Russia deployed thermobaric weapons during their assault on Grozny during the late 1990s, while the Russians and their Syrian client regime both employed Russian-supplied thermobarics to attacks civilians in the course of Syria’s catastrophic civil war. The United States has regularly deployed thermobaric weapons in Iraq and Afghanistan, to include employing the “mother of all bombs” against an ISIS-K facility in 2017.

Thermobaric Weapons as Means of Warfare

It is an established principle of international law that “the right of the parties to an armed conflict to choose methods or means of warfare is not unlimited.” In particular, thermobarics appear to implicate the Convention on Certain Conventional Weapons (CCW), a treaty ratified by both the Russian Federation and Ukraine that regulates the deployment of incendiary weapons. Meanwhile, other instruments and custom prohibit the use of weapons that have indiscriminate effects. Some critics also argue that thermobarics are inherently indiscriminate weapons, and thus unlawful in all circumstances.

Thermobarics Are Not Incendiary Weapons

At first glance, thermobaric weapons might seem to run afoul of Protocol III of the CCW, which limits the use of incendiary weapons. The Protocol defines incendiary weapons as “any 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) excludes from that definition munitions “which may have incidental incendiary effects” and “[m]unitions designed to combine … blast … with an additional incendiary effect, such as … explosive bombs and similar combined-effects munitions in which the incendiary effect is not specifically designed to cause burn injury to persons.”

Thermobaric weapons are not incendiary weapons as defined by the CCW. While they depend on a chemical reaction, they are not “primarily designed to set fire … or to cause burn injury.” Instead, thermobarics are primarily designed to generate blast and pressure. While flame and heat are employed as the most efficient way to generate that blast and pressure, thermobaric weapons are not primarily designed to set fire or cause burns. Burns to persons are an incidental effect, and thus thermobarics are not properly categorized as incendiary weapons.

Thermobarics Are Not Inherently Indiscriminate Weapons

Critics charge that thermobarics have an “indiscriminate and uncontained nature.” While these weapons may be subject to potentially unlawful methods of employment (addressed below), thermobarics themselves are not inherently indiscriminate. Inherently indiscriminate weapons are those that by their nature are incapable of complying with the principles of distinction and proportionality. Vanishingly few weapons fit that category. A footnote to the U.S. DoD Law of War Manual provides as an example a proposal to strap incendiary devices to a flock of live bats, noting that “[i]f that is the best example that can be given of an inherently indiscriminate weapon, there are not very many inherently indiscriminate weapons in existence.” (para. 6.7.1, FN 151).

In the Nuclear Weapons advisory opinion, the International Court of Justice (ICJ) acknowledged that LOAC “at a very early stage, prohibited certain types of weapons … because of their indiscriminate effect,” admitted that “the use of such weapons in fact seems scarcely reconcilable” with LOAC’s requirement for discrimination, and yet declined to hold that the effects of nuclear weapons were so indiscriminate as to render their use unlawful. Given that the ICJ did not brand nuclear weapons, with their uniquely awful characteristics including the “ability to cause damage to generations to come,” as inherently indiscriminate, thermobaric weapons are almost certainly not per se unlawfully indiscriminate.

State practice also argues thermobaric weapons are not by their nature indiscriminate. Article 36 of Additional Protocol I, to which both the Russian Federation and Ukraine are party, requires that States conduct a review to ensure new weapons are not by their nature unlawful, an obligation undertaken by the United States as a matter of policy. (para 6.2). The U.S. review process includes assessment of whether “the weapon is inherently indiscriminate,” and similarly the Russian Federation forbids employment of “weapons of an indiscriminate character.” The fact both nations employ thermobaric weapons indicates that neither finds them per se indiscriminate and there is no record in the International Committee of the Red Cross customary international law database of a nation expressly adopting that position. Although the United Kingdom reportedly agonized over the legality of thermobaric weapons, no nation has declared them per se unlawful.

Thermobarics as Method of Warfare

While thermobaric weapons may not be unlawful as a means of warfare, the method of their employment could still run afoul of international law. In both international and non-international armed conflicts, any “attack” implicates the LOAC targeting regime. The most common criticisms of thermobaric weapons implicate rules governing distinction, proportionality, and the principle of humanity/prevention of unnecessary suffering.

Distinction and Proportionality

The LOAC rules governing distinction limit attacks to lawful military objectives. Critics charge that thermobarics “are prone to indiscriminate use, especially in or near populated areas.” The same argument can be (and recently has been) advanced against almost any ranged explosive weapon, but ultimately the law of war allows most attacks against military objectives unless the incidental harm anticipated would be excessive compared the military advantage expected to be gained. This analysis applies to thermobaric weapons no differently from any other method of warfare.

Unnecessary Suffering

Humanity, “defined as the principle that forbids the infliction of suffering, injury, or destruction unnecessary to accomplish a legitimate military purpose” is a cornerstone of LOAC. (para 2.3).  This rule dates to the 1868 St. Petersburg Convention’s condemnation of “arms which uselessly aggravate the sufferings” of combatants, codified by the 1899 Hague Regulations prohibition on “arms, projectiles, or material of a nature to cause superfluous injury,” refined by the 1907 Hague Regulations prohibition on “arms, projectiles, or material calculated to cause unnecessary suffering.” The 1993 CCW revises the language back to the 1899 prohibition on means of warfare “of a nature to cause superfluous injury or unnecessary suffering,” and Article 35 of AP I echoes this of a nature framing.

Commentators have pointed out that the subtle linguistic shift has outsized impacts—a ban on weapons of a nature to cause unnecessary suffering is a much stricter standard than one only on weapons calculated to cause such harm. The United States considers the calculated to standard to be a “clearer expression of the intent of governments to focus on design and intended purpose rather than every remote possibility of weapon injury,” and evaluates weapons for humanity under the calculated to standard. (para 6.6.1, FN 124).

The overpressure from a thermobaric detonation has been graphically described as a force that “just blows your lungs out of your mouth. It kind of turns you inside out.” A more sober, but no less sobering CIA analysis notes that people not obliterated outright “are likely to suffer many internal, and thus invisible injuries, including burst eardrums and crushed inner ear organs, severe concussions, ruptured lungs and internal organs, and possibly blindness.” It is beyond dispute that blast weapons have the capacity to inflict horrific suffering, but as a matter of law they are not calculated to inflict suffering beyond that justified by military necessity.

Conclusion

It is difficult to watch Russian forces shell civilians and attack maternity hospitals in the course of an unlawful war of aggression. The use of thermobaric weapons is a tempting addition to the bill of particulars against Russia, but it is important not to conflate the blatant illegality of Russian methods in Ukraine with the lawfulness of the means of warfare they have employed.

Thermobaric weapons are designed to destroy an enemy—quickly, over a wide area, and even if that enemy is behind cover. So long as the weapons are targeted at lawful military objectives, accompanied by feasible precautions to protect civilians and civilian objects, and employed in accordance with the principle of proportionality the fact that they inflict necessary suffering is not a bar to their lawful use.

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Major Matt Montazzoli is an active duty Army judge advocate, currently assigned as Operational Law Attorney (Military Personnel Exchange Program), 1st Division / Deployable Joint Force Headquarters, Australian Army at Gallipoli Barracks, Brisbane.

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Footnotes

[i] With apologies to engineers, while there are some arcane distinctions between true thermobarics and other forms of enhanced blast weapons, the differences are not significant to this legal analysis and I have chosen to employ thermobaric as a blanket term.

Photo credit: Vitaly V. Kuzmin via Wikimedia Commons