Combatants in armed conflict may choose to modify weapons in the field for a variety of reasons. They may modify them to improve their reliability. They may alter them to increase their range or accuracy. Or they may adapt them to novel purposes. During the Russia-Ukraine War, both Russia and Ukraine have used field-modified weapons to stunning effect. For example, Ukrainian forces have converted commercial drones to carry grenades that are dropped like bombs from the air. The ability to adapt and innovate using materials on hand proved crucial to slowing Russia’s initial offensive, and nearly a year into the conflict, Ukrainian forces continue to experiment with new weapon designs.

The successful use of modified weapons has highlighted the value of initiative and creative problem solving by soldiers at the tactical level. Weapons modified in the field, however, can implicate important rules regulating the means and methods of warfare. This post explores the phenomenon of field-modified weapons, the obligation to review their lawfulness, and the personnel responsible for conducting those reviews.

Weapons Review Requirement

Combatants do not have unfettered discretion to choose their means and methods of warfare in armed conflict. This “basic tenet” of the law of armed conflict lies at the heart of international weapons law. General principles of the law of armed conflict, including the principles of humanity and distinction, may prohibit the use of certain weapons, such as those that cause superfluous injury or unnecessary suffering or that are by their nature indiscriminate. Weapons may also be prohibited by specific treaties. Early weapons bans outlawed particularly devious instruments, including barbed weapons and poison. Current international agreements govern a host of modern weapons, such as landmines, cluster munitions, incendiary weapons, expanding bullets, biological weapons, non-detectable fragments, chemical weapons, and blinding lasers.

States rely on various mechanisms to ensure compliance with international weapons law, including treaty-based arrangements and municipal legislation. Legal reviews of new means and methods of warfare also serve as an important check for lawfulness during the development and procurement of new weapons. Importantly, these reviews may also be required when weapons already adopted by a State are later modified in the field. For States party to Additional Protocol I, the legal review requirement is outlined in Article 36, which states,

In the study, development, acquisition or adoption of a new weapon, means or method of warfare, a High Contracting Party is under an obligation to determine whether its employment would, in some or all circumstances, be prohibited by this Protocol or by any other rule of international law applicable to the High Contracting Parties.

Whether the obligation to conduct weapons reviews is also required as a matter of customary international law is unclear. Some sources suggest customary international law includes a weapons review obligation, albeit one narrower in scope than Article 36 (see, e.g., Boothby at 342 and Air and Missile Warfare Manual at 84). Others maintain there is insufficient evidence to find such an obligation under customary international law (see, e.g., Jevglevskaja). The United States subscribes to the latter view. In its response to the Stockholm International Peace Research Institute’s (SIPRI) questionnaire on the Article 36 review process, the United States explained that while it “views the review of the legality of weapons, means, and methods of warfare as a best practice for the implementation of customary and treaty law relating to weapons, means, and methods of warfare, [it] does not consider customary law to require these reviews as such” (U.S. DoD Response to SIPRI at 40). With respect to Russia and Ukraine, determining whether a customary obligation exists is unnecessary because the provisions of Article 36 apply to both States by virtue of their status as parties to Additional Protocol I.

Notably, the law of armed conflict does not define the terms “means of warfare” and “methods of warfare.” “Means of warfare” is generally understood to refer to the weapons or devices used to conduct war, and “methods of warfare” normally describes how belligerents conduct war (DoD Law of War Manual, § 5.1.1; ICRC Commentary to AP I, ¶1957; see also Biller & Schmitt at 195-203). W. Hays Parks suggested that while “weapons” and “means” are generally regarded as synonymous, “[m]eans of warfare also may refer to tactical means utilized to accomplish a military mission or achieve a military objective” (Parks at 118-119). By way of example, Parks offered that “starvation of an enemy nation is a method of warfare. Destruction of crops or execution of a blockade are two means by which the method may be accomplished” (Parks at 119).

Weapons Review Authority

Article 36 also does not specify how new weapons reviews must be carried out. The Commentary to Additional Protocol I observes that Article 36 “implies the obligation to establish internal procedures,” but those “internal procedures” are not further described (¶ 1470). A handful of States—for example, the United States, the Netherlands, Sweden, Belgium, Norway, Australia, Canada, Denmark, and Germany—have established formal review mechanisms, but their examples are the exception rather than the rule. Each of these State mechanisms also differs, reflecting a diversity of approaches.

Whatever procedures a State may prescribe, the weapons review process has the potential to become strained during armed conflict, particularly as new and modified weapons are submitted for review. Given the need for urgency of action in combat, how can a State review the lawfulness of field modifications in a way that is both timely and consistent with its international legal obligations during an active armed conflict?

New weapon reviews need not be administratively burdensome, but they must be completed systematically and meet a minimum standard of review. They must also be conducted at a high level of authority. W. Hays Parks suggests that the negotiating history of Article 36 establishes the obligation as a national responsibility. As such, “the legal review of a new weapon or munition must be carried out at the national government level, such as the Ministry of Defence, the Ministry of Foreign Affairs, or a combination thereof” (Parks at 108).

The United States, for example, requires that legal reviews take place at the military department level—that is, by individual branches of the armed services (DoD Law of War Manual, §6.2). By policy, the service Judge Advocates General or their designees perform this function (AR 27-53, para. 4(f); AFI 51-401, para. 2.1.2; SECNAVINST 5000.2G, encl. 18, para. 2). Australia also assigns responsibility for conducting reviews to a senior military lawyer: the Director General of the Australian Defence Force Legal Services (UN CCW GGE, Australia Submission, para. 4). Other States similarly commit the evaluation process to national-level authorities, either in the form of individual reviewers, review by committee, or through the exercise of executive veto power (see Daoust et al. 354-60; McClelland at 403).

Field-Modified Weapons

Given the role that national agencies play in the “study, development, acquisition or adoption” of new weapons, it is appropriate that new weapons reviews should occur at the national level. Presumably, national-level reviewers also possess the requisite knowledge and expertise to conduct proper legal reviews (see Parks at 100-102). Weapons modified in the field, however, bypass formal study, development, acquisition, and adoption processes in a way that can promote a sense of separation from the norm. Accomplished by soldiers at the tactical level, often operating far from higher headquarters, the field modification of weapons can assume an air of urgency that may seem to demand an exception to ordinary review and oversight processes. In Ukraine, for example, frontline units that engage in modifying drones often face less bureaucracy before they may test and deploy their weapons. According to one report, the modifications can then be quickly “shared with other drone units in chat groups before being used in the field, with little oversight.”

Field-modified weapons should be reviewed for legal compliance for the same reasons new weapons are subjected to legal review. Even seemingly minor changes to otherwise lawful weapons can result in noncompliance with a State’s international legal obligations or the law of armed conflict. This concern is reflected in the United States’ restrictive policy regarding the modification of weapons. The DoD Law of War Manual states, “For a variety of reasons, DoD practice has been not to permit the modification of weapons without proper authorization” (§6.3.2). For the United States, “proper authorization” means permission granted by the original weapons review authority. In the U.S. Army, for example, this would be the Judge Advocate General’s designee. U.S. Army regulations state, “If the weapon or weapon system is changed once it has been fielded such that it is no longer the same system or capability described in the legal review request … then the legal review is no longer valid and a new legal review from the TJAG [the Judge Advocate General] designee must be obtained …” (AR 27-53, para. 6(f)).

Case Example: Drone-Delivered Grenades

In Ukraine, the alteration of common grenades to be drone delivered has proved dramatically effective. According to various reports, Ukrainian forces have rigged commercial off-the-shelf drones to carry grenades and modified those grenades to drop like bombs and explode on impact. These modest weapons have neutralized Russian tanks and other critical military targets throughout the Ukrainian theater (see, for example, herehere, and here). Determining whether these improvised weapon systems must be reviewed under Article 36 is an important question of compliance under the law of armed conflict.

Ukraine’s improvised systems raise multiple issues for consideration. First, are the drone delivery systems lawful weapons (means of warfare)? Second, are the modified grenades lawful weapons? And lastly, is the delivery of the modified grenades by drone a lawful method of warfare?

Drone Modifications

Ukraine is credited as one of the first States to modify commercial drones for kinetic strikes. On the one hand, commercial off-the-shelf drones are not designed to be weapons or weapon systems and, therefore, arguably need not be legally reviewed. Professors Jeffrey Biller and Michael Schmitt explain that weapon systems are “systems specifically designed for effectuating an attack using the weapon in question. That being so, it would be logically impossible to have a weapon system that was not designed to employ a weapon” (at 203). By way of example, they note that “a standard truck fitted with a rocket launcher or filled with explosives for a suicide attack is not a weapon system, as the truck was not designed to facilitate such attacks” (at 203). Accordingly, commercial off-the-shelf drones acquired by Ukrainian forces need not be reviewed as new weapons or means of warfare upon procurement or acquisition.

On the other hand, commercial off-the-shelf drones modified to carry air-delivered grenades could be viewed as weapon systems, necessitating their review under Article 36. The U.S. Army defines a weapon system as “the weapon itself, combined with those components required for its operation that permit the weapon to kill, injure, disable or incapacitate enemy personnel or to destroy, damage, or incapacitate enemy facilities, equipment, property or materiel,” including “all munitions such as projectiles, small arms, mines, and explosives” needed to accomplish these outcomes (Army Reg. 27-53 at 9). Arguably, a commercial drone that is subsequently designed and modified to carry a grenade constitutes part of a weapon system that must be legally reviewed. Alternatively, pairing drones with air-delivered grenades may represent a new method of warfare, triggering a separate basis for a legal review. In general, the legal review would address and analyze a variety of factual and legal considerations, such as the purpose or mission of the weapon or weapon system (i.e., military necessity), relevant treaty law, and the weapon or weapon system’s compliance with the principles of humanity and distinction.

Grenade Modifications

Like altered drones, modified grenades must also be evaluated for lawfulness. Reports indicate that Ukrainian forces have altered a variety of common grenades, including Soviet-era F-1 anti-personnel hand grenades, RGD-5 anti-personnel fragmentation grenades, RKG-3 anti-tank grenades, VOG-17 fragmentation grenades, and German DM51 fragmentation grenades for air delivery (see, for example, herehere, and here). One modification involves encasing a grenade in a “bait bomb”—a plastic container ordinarily filled with bait by fishermen—and setting its fuse to explode on impact. In other instances, Ukrainian forces have converted grenades themselves into bombs by adding attachments, including 3D-printed fins and other parts.

Determining whether alterations like these would subject the modified grenades to a follow-on weapons review will depend on the nature of the changes. If the modifications alter the mission, purpose, or performance parameters of the grenades to the extent that they can no longer be considered the same weapons already reviewed under Article 36, then they must undergo a new legal review. Otherwise, no additional review would be necessary. The conveyance of the grenades by air, on the other hand, might necessitate an Article 36 review of the drone-delivered grenades as a new method of warfare.

Drone Delivery of Grenades

The International Committee of the Red Cross (ICRC) has observed, “A weapon or means of warfare cannot be assessed in isolation from the method of warfare by which it is to be used” (Guide to the Legal Review of New Weapons at 10). Under Article 36, the reviewing State’s obligation is to analyze whether the employment of the weapon for its “normal or expected use” would be prohibited “under some or all circumstances” (ICRC Commentary to AP I, ¶ 1469). The Commentary explains that “[a] State is not required to foresee or analyse all possible misuse of a weapon for almost any weapon can be misused in ways that would be prohibited” (¶ 1469).

An Article 36 analysis of drone-delivered grenades as a potentially new method of warfare would likely focus on distinction and the prohibition against indiscriminate attacks (see AP I, art. 51(4)). Among other things, the review would evaluate whether the drone delivery of grenades can be directed at specific military objectives and whether the effects of such use can be limited. Compliance with the law of armed conflict will depend largely on the ability to discriminate using this method of warfare.

Legal Review of Field-Modified Weapons

As discussed above, the practice of States has been to reserve new weapons reviews to national-level authorities who evaluate new weapons for compliance with the State’s treaty obligations and the law of armed conflict. Modified weapons should undergo legal review for the same reasons. However, not all modified weapons will require a de novo weapons review. Weapons altered in ways that do not fundamentally change the weapon’s intended mission, purpose, or performance parameters arguably should not require a subsequent new weapons review. Rather, a de novo national review should be required when alterations raise doubt about a modified weapon’s legality. In other words, modified weapons should not be treated presumptively as novel weapons and subjected to a blanket requirement for review. Instead, they should first be carefully evaluated to determine whether a subsequent Article 36 review is warranted. When not warranted, modified weapons should be lawfully available for use by the State in the armed conflict.

Not all modifications made to a weapon in the field will jeopardize the initial determination of legality under Article 36. For example, a tweak that increases the durability of a firearm (see, for example, the WWII-era M3 “Grease” submachine gun) but does not significantly change the weapon’s mission, purpose, or performance parameters should not require a de novo weapons review. Similarly, a modified artillery round that modestly increases the effective range of a howitzer but does not otherwise alter its expected use should not necessarily be subjected to a follow-on Article 36 review. In contrast, a modification that changes the intended mission or purpose of a weapon or significantly alters the weapon’s performance characteristics should trigger a de novo review at the national level. For example, an artillery round that has been modified to scatter sub-munitions should be subjected to a new Article 36 review.

Some States, such as the United States, strictly prohibit the modification of weapons without proper authority, although prior authorization is not required as a matter of international law. A restrictive approach can allow a State to monitor and control the weapons employed by its forces more closely in combat. This would mean that all weapons fielded by the State’s armed forces—including those modified by frontline units—are likely to undergo a national-level review. Strict policy limitations on modifications, however, could have a downside. Requiring forces to obtain authorization prior to modifying a weapon, rather than prior to the weapon’s use, can stifle the type of innovation and initiative that Ukrainian defenders have displayed throughout the Russia-Ukraine War.

The experience of Ukrainian forces during the Russia-Ukraine conflict suggests an all-or-nothing approach to weapons modifications and reviews would be impractical and unrealistic for most States. To remain relevant and effective, the requirements of Article 36 cannot be understood to require a de novo national review of any and all weapons modified in the field during armed conflict. While substantial changes to a weapon’s intended mission, purpose, or performance parameters should be grounds for a subsequent new weapons review, lesser changes could potentially be evaluated for legal compliance at lower levels of command, echelons below national review authorities.

Preliminary Review by Legal Advisers in the Armed Forces

A preliminary review conducted by a qualified legal adviser could help determine whether a subsequent new weapons review is legally required for a field-modified weapon. Potentially, that responsibility could be entrusted to a legal adviser operating with the armed forces in the field. Article 82 of Additional Protocol I, which addresses the role of legal advisers in the armed forces, suggests military legal advisers should be qualified to conduct this type of preliminary review. Article 82 provides,

The High Contracting Parties at all times, and the Parties to the conflict in time of armed conflict, shall ensure that legal advisers are available, when necessary, to advise military commanders at the appropriate level on the application of the Conventions and this Protocol and on the appropriate instruction to be given to the armed forces on this subject.

Professor Yoram Dinstein points out that the language of Article 82 is “somewhat unusual” because it “combines a clear-cut obligation (immanent in the phrase ‘shall ensure’) with a certain degree of flexibility (derived from the unspecified reference to ‘the appropriate level’ and the qualifying words ‘when necessary’)” (Dinstein at 919). The Commentary to Additional Protocol I acknowledges that State views on when and where to appoint armed forces legal advisers will diverge. The Commentary explains,

[T]he content of the obligation and the extent of the measures to be taken can certainly vary from one country to another, depending on the importance of the role which these legal advisers are called upon to play. Some may wish to appoint these advisers at all—or nearly all—levels of command, while others intend to appoint them only at the headquarters of large units and at military academies, and still others only envisage their participation in exceptional situations (para. 3344).

The review of weapons is clearly a “choice of arms” question that falls within the purview of the legal adviser. The DoD Law of War Manual acknowledges this by ascribing responsibility for weapon reviews to qualified legal advisers in a section dedicated to the “Role of Judge Advocates and Legal Advisers” (§ Because new weapons reviews are considered a national responsibility, however, legal advisers at lower echelons do not have authority to conduct these reviews. On the other hand, determining whether a weapon modified in the field requires a new Article 36 review presents a different question. Arguably, that determination may fall within the competence of lower-level legal advisers in the armed forces.

The Commentary to Additional Protocol I notes that Article 82 itself “goes so far as to define in broad terms the qualifications which such legal advisors must possess, since they must be capable of advising, when necessary, the military commanders concerned, on the application of the Conventions and the Protocol” (¶ 3344). Enabling these legal advisers to give an initial appraisal of modifications—a preliminary review—would also reflect a more realistic and practical approach to the assessment of weapons altered in the field. Timely analysis and decision-making have always been prized in war, and these advisers could provide commanders the legal advice needed to respond decisively and lawfully in the crucible of armed conflict.

This approach appears to have gained some support in the cyber context. The Tallinn Manual 2.0 notes that a majority of its expert drafters believed a formal legal review was not required before the employment of a cyber capability (Tallinn Manual 2.0 at 465; see also Biller & Schmitt at 220). Instead, “the advice of a legal advisor at the relevant level of command was deemed … to suffice in lieu of a formal legal review” (at 465). The manual further explains that “[i]f a means or method of cyber warfare is being developed for immediate operational use, the lawyer who advises the commander planning to use it will be responsible for advising whether the cyber weapon or the intended method of its use accord with the State’s international law obligations” (at 466). Professors Biller and Schmitt point out that operations conducted at the “speed of cyber” could nevertheless prove “a significant obstacle to the rendering of meaningful reviews,” even with legal advisers embedded in mission planning cells (Biller & Schmitt at 220). However, the acknowledgment that legal advisers in the field can and should be involved in the review of weapons suggests they could play a greater role in the evaluation of field-modified weapons and their subsequent use.


The United States’ restrictive approach to weapons modifications comes with many benefits, but it can dampen the drive to improvise and innovate on the battlefield. The requirement for national-level authorization serves as an important check before a weapon is modified, and national-level review provides visibility on all weapons changes and consistency in their evaluation. Moreover, these reviews can be conducted rapidly. Parks, who served as the U.S. Army’s weapon review authority from 1979-2003, said he knew of

no backlog or delay in providing weapons reviews as a result of the nature of the legal review process utilised by the DoD. In cases where urgency has been an issue and where the weapon has been reviewed previously, a verbal ‘review’ has been provided, followed up within a day or two by the written legal review (Parks at 110-111).

Other States similarly allow for the fast-tracking of review decisions based on urgent operational needs (see, e.g.,Boulanin at 23).

Not every State will control the modification process as tightly as the United States—and none is required to. However, States party to Additional Protocol I, including Russia and Ukraine, are responsible for reviewing the lawfulness of new weapons—including weapons that have been altered in ways that change their intended mission, purpose, or key performance characteristics—in accordance with Article 36. Determining when a field modification requires a de novo Article 36 review is a question that lower-level legal advisers are arguably competent to answer. It is also one that is likely to become more common as the next generation of arms, to include autonomous weapons, are enhanced by new technologies. Adoption and publication of clear weapon review programs and policies by States, accompanied by policies and legal assessments applicable to field modifications, will prove essential to maintaining longstanding and proven law of armed conflict limits on the means and methods of warfare.


LTC Ronald Alcala is an Associate Professor and Associate Dean for Strategy and Initiatives at the United States Military Academy at West Point. He is also Managing Editor of Articles of War.