In the previous article of our series on select principles of international humanitarian law (“IHL”), also referred to as the law of armed conflict (“LOAC”), we discussed the protection of combatants from unnecessary suffering.
In this Article, we provide an introduction to the obligation of states to conduct legal reviews of weapons, means and methods of warfare under Article 36 of Additional Protocol I (AP I) to ensure their compliance with international law.
To enable readers to quickly grasp the essential treaty provisions, we have once again highlighted key passages.
Estimated reading time: 10 minutes
I. What is the Purpose of Weapon Reviews?
In 1863, the Parties to the Saint Petersburg Declaration took an important step. They renounced the use of explosive projectiles weighing less than 400 grams in wartime. They did so because they considered such projectiles to cause unnecessary suffering.
More importantly, the Parties also reserved the right to regulate future improvements in the armament of troops. They intended “to conciliate the necessities of war with the laws of humanity”.
Since then, international treaties have prohibited a significant number of weapons. These include biological, chemical and conventional weapons, as well as anti-personnel mines and cluster munitions.
As the U.S. DoD has observed, most current weapons are still not illegal per se but only when misused in an unlawful manner. However, the right of belligerents to adopt means of injuring the enemy is not unlimited (Article 22 of the Hague Convention IV of 1907 and Article 35 para. 1 of Protocol One to the Geneva Conventions of 1949 (AP I)
States conduct legal weapon reviews under Article 36 AP I to ensure compliance with international law. They must design, develop, acquire and use weapons in a manner that respects those legal obligations.
II. What is the Legal Basis for Weapon Reviews?
Article 36 AP I provides that
“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 Party.”
Moreover, States that have become parties to Additional Protocol I have implemented domestic regulations governing weapon reviews. Some States that have not ratified the Protocol have adopted similar domestic frameworks. We will address select examples of these hereinafter.
III. Who Must Carry Out Weapon Reviews?
States that Have Become Parties to Additional Protocol I
Under Article 36 AP I, only a “High Contracting Party” is under an obligation to conduct weapon reviews.
As the ICRC has remarked in its commentary on Article 100 AP I: “the expression “High Contracting Parties” covers States which have become Parties to the Protocol by ratification, accession or succession.”
Other States
Article 36 binds all parties to Additional Protocol I. However, current State practice does not establish a general obligation to conduct legal weapon reviews under customary international law.
In military practice, legal departments within Ministries of Defence usually conduct weapon reviews. Where necessary, committees or other expert bodies provide these departments with technical, operational and medical expertise.
IV. What is a “Weapon”?
Article 36 AP I does not provide a definition of “weapons” but takes the term as given. Remarkably, not even the Convention on Conventional Weapons (CCW) defines what constitutes a weapon.
Therefore, we must turn to other legal frameworks and state practice. These sources clarify the scope of this common but not self-explanatory term.
The ICRC Guide and HPCR Manual
Section 1.1 of the ICRC Guide addresses the scope of Article 36 AP I. It explains that Article 36 AP I covers:
“weapons of all types – be they anti-personnel or anti-materiel, “lethal”, “non-lethal” or “less lethal” and weapon systems”.
Experts developed the HPCR Manual (also referred to as the “AMW Manual”) during meetings at Harvard University to provide an accessible source of international law applicable to military operations involving air and missile warfare. Because practitioners now regard it as a valued resource for interpreting LOAC terminology, this article also refers to it here.The HPCR Manual considers a “weapon” as
“a means of warfare used in combat operations, including a gun, missile, bomb or other munitions, that is capable of causing either injury to, or death of, persons; or damage to, or destruction of, objects.”
State Practice
Australia’s Guide to the Legal Review of New Weapons, Means or Methods of Warfare defines a weapon as
“any arms, munition, device, whether tangible or intangible, designed or intended to be used to (1) cause injury to, or death of, persons; or (2) damage to, destruction or neutralisation of objects”
The express inclusion of both tangible and intangible arms in this definition supports a modern understanding of weapons in contemporary warfare, which many other states have yet to adopt.
Germany’s central service regulation on the review of new weapons, means and methods of warfare provides a more traditional, physical definition of weapons as
“objects that are intended or suitable for killing or injuring people, or for eliminating or reducing their ability to attack or defend themselves, and/or for destroying or damaging objects.”
The U.S. DoD Department of the Navy has defined a “weapon or weapon system” similarly as
“all arms, munitions, materiel, instruments, mechanisms, devices, and those components required for their operation, that are intended to have an effect of injuring, damaging, destroying, or disabling personnel or property, to include non-lethal weapons.”
Common Properties of Weapons
All of these definitions clarify that not only traditional arms such as rifles, grenades or bombs qualify as weapons. They also include means that designers originally developed for other purposes but that belligerents may nevertheless use as weapons.
Moreover, these definitions share a core requirement: the instrument must be capable of causing physical injury or death, or the destruction of objects.
V. What are “Means and Methods of Warfare”?
Means of Warfare
At first sight, one may assume that “means of warfare” must differ from “weapons”, otherwise Article 36 AP I would not address them separately. The German interpretation appears to support this view by defining means of warfare as
“objects that, without being weapons, directly affect one’s offensive or defensive abilities.”
However, legal scholarship and state practice more commonly treat “means of warfare” as an umbrella term and often use it as a synonym for weapons and related systems. As such they stand for
“weapons, weapon systems or platforms employed for the purposes of attack” (HPCR Manual),
and comprise
“weapons as well as weapon systems, which includes all related equipment, materials, and delivery mechanisms necessary for a weapon to function as intended.” (Australia’s Guide to Weapon Reviews).
Methods of Warfare
Germany views methods of warfare as
“Plans, concepts, or doctrines for military actions intended to disrupt enemy military operations and capabilities or to support one’s own military operations and capabilities.”
Australia understands methods of warfare as
“the manner in which weapons and certain capabilities are used in warfare to harm, destroy, or neutralise the enemy.”
The HPCR Manual offers a more operational definition for methods of warfare:
“attacks and other activities designed to adversely affect the enemy’s military operations or military capacity, as distinct from the means of warfare used during military operations, such as weapons. In military terms, methods of warfare consist of the various general categories of operations, such as bombing, as well as the specific tactics used for attack, such as high altitude bombing.”
According to the renowned legal scholar William Boothby
“Methods of warfare consist of the ways in which weapons are used in hostilities.”
Michael N. Schmitt who needs no introduction expressed this in operational terms by stating that:
“methods of warfare do not comprise equipment, but rather the tactics, techniques, and procedures (TTP) for carrying out military operations involving the conduct of hostilities.”
VI. When Are Weapons “New” Within the Meaning of Article 36 AP I?
Not all states that have published guidance on weapon reviews have addressed this question.
According to the ICRC Commentary on Article 36 AP I and the ICRC Guide, a “new” weapon is “a weapon which the State is intending to acquire for the first time, without necessarily being “new” in a technical sense”.
The German Ministry of Defence has adopted the essence of this definition in its central service regulation. It provides that “new” means “newly developed, significantly modified or existing, but not yet introduced into the Armed Forces”.
This definition is practical and reasonable. It reflects the reality that weapons, as well as means and methods of warfare, are subject to frequent innovation. Such changes can significantly affect their legal compliance.
It also follows from this definition that states cannot free themselves from their obligations under Article 36 AP I by arguing that weapons are not “new” merely because they have already been on the market.
Furthermore, the German service regulation reflects the widely accepted understanding that another State’s prior weapon review does not relieve a state of its obligations under Article 36 AP I.
VII. How must States carry out Weapon Reviews?
Article 36 AP I does not prescribe a specific methodology for conducting weapon reviews. Consequently, state practice is not uniform.
As of February 2026, 175 States have ratified or acceded to Additional Protocol I. However, only 17 States have provided publicly available information on how they conduct weapon reviews.This limited transparency is partly due to the fact that information on troop armament has always been sensitive, as it can provide adversaries with a military advantage.
At a minimum, a weapon review must determine whether the weapon under consideration:
(1) Violates an arms treaty that renders its employment generally unlawful. Relevant treaties notably include the
(2) May cause superfluous injury or unnecessary suffering
(3) Operates in an inherently indiscriminate manner because its design prevents operators from directing it at a specific military objective or because its foreseeable effects cause excessive incidental civilian harm.
Additional Aspects to Consider
An Article 36 review should also assess whether the weapon:
- May cause widespread, long-term and severe damage to the natural environment (Article 35 para. 3 AP I)
- Raises concerns under the Martens Clause.
Moreover, weapon reviews are not purely legal exercises. They require interdisciplinary assessment of legal, military, technical and medical aspects.
Consideration of Evolving State Practice
Given the absence of harmonised international procedures, weapon reviewers should remain informed about best practices suggested by the ICRC and evolving state practice.
Australia’s Guide is among the most comprehensive publicly available documents and deserves special mention. Australia prepared the Guide to accommodate future weapons that do not rely on kinetic force, and it has the potential to shape similar documents in other States that, like Australia, have become parties to Additional Protocol I.
Similar to the U.S. Law of War Manual, the Australian Guide also provides a helpful summary of specific prohibitions and restrictions that reviewers must examine during a legal weapon review.
Limitation of Weapon Reviews
As the ICRC has noted in its Guide, the legality of a weapon depends not only on its design and intended purpose but also on its expected use.
At the same time, Article 36 AP I requires states to assess only the normal use of a weapon. They need not
“foresee or analyse all possible misuses of a weapon for almost any weapon can be misused in a way that would be prohibited” (ibid. at 1.1, p. 10).
If such normal use is irreconcilable with international law, weapon reviewers must advise against it. This may lead to limiting permissible uses or requiring a redesign that effectively prevents unlawful use.
Timing of the Review
Article 36 AP I makes clear that states must review new weapons at the study and development stage, before acquisition or adoption.
Conducting reviews only pro forma immediately before introduction into service can result in costly modifications and severely restrict lawful use.
States must also re-examine weapons that undergo significant modification.
Weapon reviews are therefore not a one-time exercise. They must accompany a weapon throughout its lifecycle until decommissioning.
Documentation of Weapon Reviews
One can hardly overstate the importance of documentation. Without proper records, a state may be unable to demonstrate compliance with Article 36 AP I and/or domestic regulations.
Documentation should begin at the design phase. At a minimum states should record:
- Design decisions affecting compliance
- Legal assessments undertaken
- Reasons for limiting or refraining from review.
In our next Article of this series, we will address the legal review of “cyber weapons” which are increasingly seen as cyber capabilities. Cyber capabilities have emerged as a distinct category of methods of (cyber) warfare. They can be as effective as, or even more effective than, conventional weapons or means that rely on kinetic force and/or physical effects.
If you would like to discuss particular aspects of designing military AI systems for IHL compliance or if you have any questions on specific LOAC principles, get in touch with me by email or just give me a call.