What Weapons Are Not Allowed in War? A Guide to the Laws of Armed Conflict

There is no single list of weapons banned in every war. International law prohibits certain weapons outright, restricts others, and requires all weapons to be used according to rules protecting civilians and limiting unnecessary suffering. The rules can depend on which treaties the countries involved have accepted and whether a conflict is international or internal. Understanding these distinctions helps explain why some weapons are illegal by design while others may be lawful in one use and unlawful in another.

How The Laws Of War Regulate Weapons

The laws of armed conflict—also called international humanitarian law—regulate how parties fight. Their central principles include distinction, which requires combatants to distinguish military targets from civilians; proportionality, which prohibits attacks expected to cause excessive civilian harm relative to the anticipated military advantage; and precautions, which require feasible steps to reduce civilian risk.

These rules apply regardless of whether a weapon is specifically named in a treaty. A weapon may be prohibited because its effects cannot be directed at a military target or because it causes superfluous injury or unnecessary suffering. The International Committee of the Red Cross (ICRC) summarizes these principles in its [overview of international humanitarian law](https://www.icrc.org/en/law-and-policy/international-humanitarian-law).

Specific treaties add further restrictions. The relevant rules may differ according to a country’s treaty commitments, the type of conflict, and the weapon’s design and use. Consequently, “not allowed in war” can mean an outright ban, a ban on particular uses, or a general restriction that applies to every attack.

Weapons Prohibited By International Treaties

Chemical Weapons

The Chemical Weapons Convention (CWC) prohibits countries that are parties from developing, producing, acquiring, stockpiling, retaining, transferring, or using chemical weapons. It also requires the destruction of declared chemical-weapon stockpiles and production facilities under international verification. The ban covers toxic chemicals used for their harmful effects, as well as munitions and equipment specifically designed to cause harm through those chemicals.

The treaty is administered by the Organisation for the Prohibition of Chemical Weapons (OPCW). Its [overview of the CWC](https://www.opcw.org/chemical-weapons-convention) explains the treaty’s scope and verification system. Tear gas and other riot-control agents are not automatically chemical weapons in every context: the CWC permits certain domestic law-enforcement purposes, but prohibits using riot-control agents as a method of warfare.

Biological And Toxin Weapons

The Biological Weapons Convention (BWC) prohibits its parties from developing, producing, stockpiling, acquiring, or retaining biological agents and toxins in types or quantities that have no peaceful justification, as well as weapons or delivery equipment designed to use them for hostile purposes. In practice, that rules out biological weapons intended to cause disease or death.

The BWC does not have an organization with a verification system comparable to the OPCW’s inspections under the CWC. That difference affects how compliance is monitored, but it does not make biological weapons lawful. The United Nations provides a [summary of the Biological Weapons Convention](https://disarmament.unoda.org/biological-weapons/).

Poison And Poisoned Weapons

Using poison or poisoned weapons is prohibited under longstanding laws of war, including rules reflected in the Hague Regulations. The prohibition targets poison used as a weapon, rather than every harmful substance in every setting. Chemical weapons are also covered by the more detailed CWC rules. Parties must assess the weapon’s intended purpose and effects, not simply the label attached to it.

Expanding Bullets In International Armed Conflict

The 1899 Hague Declaration concerning expanding bullets prohibits its parties from using bullets that expand or flatten easily in the human body, such as certain bullets with a hard outer covering and an exposed or cut tip. The rule is often associated with “dum-dum” bullets.

This is a treaty-specific prohibition, not a blanket ban on every bullet that deforms on impact. The relevant question includes the ammunition’s design and the treaty obligations of the countries involved. The [ICRC’s treaty database](https://ihl-databases.icrc.org/en/ihl-treaties) provides the text and status information for the Hague Declaration.

Blinding Laser Weapons

Protocol IV to the Convention on Certain Conventional Weapons (CCW) prohibits laser weapons specifically designed to cause permanent blindness to unenhanced vision—that is, ordinary eyesight or eyesight corrected with glasses—when blindness is their sole or one of their combat functions. The protocol also requires parties to take feasible precautions to avoid permanent blindness caused by other laser systems.

This is not a ban on every military laser. Lasers used for range-finding, target designation, or other purposes are not automatically prohibited, although their use remains subject to general rules protecting civilians. The distinction turns on the weapon’s design and combat function.

Weapons Restricted Rather Than Completely Banned

Landmines

The 1997 Mine Ban Treaty, also known as the Ottawa Treaty, prohibits its parties from using, developing, producing, acquiring, stockpiling, retaining, or transferring anti-personnel mines. It also requires destruction of stockpiles and clearance of mined areas, subject to treaty provisions and deadlines. The ban concerns anti-personnel mines; it does not automatically cover every type of mine or explosive device.

The treaty does not bind countries that have not joined it. The United States is not a party to the Ottawa Treaty, so the treaty’s obligations should not be described as universally binding. However, any use of mines remains subject to other applicable law, including rules requiring distinction and precautions to protect civilians. The UN’s [treaty record](https://treaties.un.org/) and the [ICRC treaty database](https://ihl-databases.icrc.org/en/ihl-treaties) can be used to check a country’s participation.

Cluster Munitions

The Convention on Cluster Munitions prohibits its parties from using, developing, producing, acquiring, stockpiling, or transferring cluster munitions. These weapons release multiple submunitions over an area, and some unexploded submunitions can remain dangerous after a battle ends. The treaty also sets requirements for stockpile destruction, clearance, and assistance to affected communities.

The convention does not bind states that have not joined it. For countries outside the treaty, the legality of a particular use must be assessed under other applicable rules, including distinction, proportionality, and precautions. A weapon’s wide-area effects and the risk posed by unexploded ordnance can be especially relevant when civilians are nearby.

Incendiary Weapons

Protocol III to the CCW restricts incendiary weapons, which are primarily designed to set objects on fire or cause burn injuries through flame or heat. It prohibits attacks with such weapons against civilians and civilian objects, and places additional limits on attacks against military targets located within concentrations of civilians.

The protocol is not a universal ban on every weapon that produces heat or fire. It excludes some munitions with incidental incendiary effects, such as certain illumination or smoke devices, from its definition. Even where a specific weapon is not covered by Protocol III, an attack must still comply with the general rules of distinction, proportionality, and precautions.

Weapons That Cause Unnecessary Suffering

International law prohibits weapons of a nature to cause superfluous injury or unnecessary suffering. This principle is reflected in the 1907 Hague Regulations and Additional Protocol I to the Geneva Conventions. It is a broad standard rather than a complete list of banned products, and its application can require legal and technical analysis.

Under Article 36 of Additional Protocol I, states that are parties must review new weapons, means, or methods of warfare to determine whether their use would be prohibited in some or all circumstances. The review considers the weapon’s expected effects and applicable law, not just its intended military advantage.

Weapons That Are Not Universally Banned

Nuclear Weapons

Nuclear weapons are not prohibited by a single treaty binding every country. The 2017 Treaty on the Prohibition of Nuclear Weapons (TPNW) bans its parties from activities including developing, possessing, using, or threatening to use nuclear weapons. Countries that have not joined the TPNW are not bound by those treaty prohibitions simply because the treaty exists.

That does not mean nuclear weapons can be used without legal limits. Any use must be assessed under applicable international law, including the UN Charter and the laws of armed conflict. The International Court of Justice’s 1996 advisory opinion concluded that the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict, while stating that it could not reach a definitive conclusion in an extreme circumstance of self-defense involving a state’s survival. The [ICJ opinion](https://www.icj-cij.org/case/95) is a key reference, but it is not a universal treaty ban.

Depleted Uranium And Other Controversial Weapons

Depleted-uranium munitions are not subject to a specific, universal treaty ban. Their use is nevertheless governed by general rules of armed conflict. Legal assessments may consider the weapon’s effects, the target, the risk to civilians, and whether the attack is proportionate and takes feasible precautions.

The same principle applies to many weapons that are controversial but not categorically prohibited by a global treaty. Their legality depends on the weapon’s properties, the way it is used, and the rules binding the parties. A weapon’s presence on a battlefield does not by itself establish that its use is lawful.

When A Weapon’s Use Becomes Illegal

A weapon that is not banned by name can still be used unlawfully. For example, an attack may violate the law if it is directed at civilians, cannot be aimed at a specific military objective, or is expected to cause excessive civilian harm compared with the anticipated concrete and direct military advantage. Failing to take feasible precautions can also make an attack unlawful.

The reverse also matters: a weapon’s lawful status does not make every use lawful. A weapon permitted in some circumstances may be prohibited in a particular location, against a particular target, or when its effects cannot be controlled. Legal reviews therefore consider both the weapon and the planned operation.

How To Check Whether A Weapon Is Banned

A careful assessment usually asks several questions:

• What does the weapon do? Its design, intended function, and foreseeable effects can determine which rules apply.
• Which treaty rules bind the parties? Treaty participation varies by country, and some agreements have limited membership.
• What kind of conflict is involved? Some treaty rules are framed for international armed conflict, while other rules apply more broadly.
• How will the weapon be used? Target selection, civilian presence, expected harm, and available precautions matter.
• Does customary international law apply? Some prohibitions or principles may bind states beyond a particular treaty’s membership, though the precise rule must be assessed carefully.

For authoritative research, treaty texts and participation records are available through the [United Nations Treaty Collection](https://treaties.un.org/), the [ICRC treaty database](https://ihl-databases.icrc.org/en/ihl-treaties), and the relevant treaty organizations, including the [OPCW](https://www.opcw.org/) and the United Nations Office for Disarmament Affairs. These sources help distinguish an outright prohibition from a restriction that applies only to certain states, weapons, or uses.

Key Takeaway

Weapons most clearly prohibited by major international agreements include chemical and biological weapons, poison, certain expanding bullets under applicable treaty rules, and laser weapons designed specifically to cause permanent blindness. Anti-personnel mines, cluster munitions, and incendiary weapons are subject to important treaty bans or restrictions, but those treaties do not bind every country. Nuclear weapons and other controversial arms are not covered by a universal ban binding all states. In every case, the way a weapon is used must also comply with the general laws of armed conflict.