International law limits the methods and means used to wage war. These restrictions apply to the type of weapons used, the way they are used and the general conduct of all those engaged in the armed conflict. The principle of distinction requires that Parties to an armed conflict distinguish at all times between combatants and military objectives on the one hand, and civilian persons and objects on the other, and accordingly attack only legitimate targets.

Methods and means
The main treaties placing limits on methods and means of waging war are the Hague Convention of 1907, the 1977 Additional Protocols to the Geneva Conventions and a series of agreements on specific weapons. The ICRC has been involved in the process of developing the law in this field.

In general terms, international humanitarian law prohibits means and methods that cause superfluous injury or unnecessary suffering. As a result certain types of weapons are not allowed and the way other weapons are used is restricted.

Specific measures to limit the use of certain types of weapons include the 1997 Ottawa Convention on the prohibition of anti-personnel mines, the 2003 Protocol on explosive remnants of war (an addition to the 1980 UN Convention on Certain Conventional Weapons) and the 2008 Dublin Treaty banning cluster bombs.

Progress is also being made on controlling the proliferation of small arms that are so lethal in many poorer countries. Parallel to bans and restrictions on unacceptable weapons, IHL also limits the use of “acceptable” conventional weapons.

The law also regulates many other methods and means of conducting armed conflicts. There are rules on the misuse of flags of identification and the treatment of mercenaries; it is forbidden to order that there be no survivors in battle. Specific rules apply to demilitarized zones and non-defended areas.

Distinction
In international armed conflict, combatants are entitled to directly participate in hostilities. In other words, they are permitted to commit lawful acts of war intended to achieve a military goal in the most effective way. The principle of distinction, however, requires that such acts of war be directed only against enemy combatants and military objectives, while preventing unnecessary and excessive damage to civilians.

Therefore, implementation of the principle of distinction demands a clear definition of persons and objects that may be legally targeted. Insofar as persons are concerned, enemy combatants are members of the armed forces of a Party to a conflict (except for medical personnel and chaplain). Persons who are not members of the armed forces are civilians and therefore shall not be the object of attack. There is one exception, however: civilians directly participating in hostilities – either individually or as part of a group – become legitimate targets of attack, though only for the duration of their direct participation in hostilities.

As far as objects are concerned, military objectives are defined through a two-pronged test: the object to be attacked must by its nature, location, purpose or use contribute effectively to the military action of the enemy and its partial or total destruction, capture or neutralization, must offer – in the circumstances ruling at the time – a definite military advantage. All objects which do not fall under the definition of a military objective are civilian objects and shall not be attacked.

IHL includes a number of corollaries to the principle of distinction in order to secure the protection of civilian persons and objects. For example, it stipulates that combatants in an international armed conflict are required to distinguish themselves from the civilian population (normally by wearing a uniform) while they are engaged in an attack or in a military operation preparatory to an attack. In addition, IHL prohibits indiscriminate attacks and provides for the principle of proportionality, which dictates that the so called “incidental loss” of civilian life and/or property should not be excessive in relation to the concrete and direct military advantage anticipated. In order to implement the restrictions and prohibitions on targeting, specific precautions must also be observed by all Parties to an armed conflict.

Finally, the rules on the conduct of hostilities also grant a specific protection to some objects, including cultural objects and place of worship (such as historic monuments), objects indispensable for the survival of the civilian population (including agricultural areas for the production of food-stuffs, crops, or drinking water installations) and works and installations containing dangerous forces (namely dams, dykes and nuclear electrical generating stations). Means and methods of warfare with the potential to cause widespread, long-term and severe damage to the environment are prohibited as they threaten the health and survival of the civilian population.