The Geneva Conventions of 1949 and their Optional Protocols of 1977 (the main treaties of International Humanitarian Law – IHL) set a range of minimum standards for the conduct of hostilities. They are based upon the fundamental principle of distinction between civilians and other protected persons, on the one hand, and those who take part in hostilities (combatants for short).
The term “civilian” refers to individuals or objects (e.g., premises) that do not have a direct role in hostilities (See Rule 5 and Rule 9 of the Study on customary international law by the International Committee of the Red Cross – ICRC). An attack against a civilian person or object is therefore generally a violation of IHL and may constitute a war crime. A person or object can however lose its civilian status if it starts making an effective contribution to military action. It would then become a legitimate military objective (and hence a target) (See Rule 10 of the Study on customary international law by the ICRC). This determination must however be unequivocal: when in doubt as to whether a school or hospital has become a military objective, there is a presumption that it retains its civilian status.
Even attacks against legitimate military targets must, however, follow two additional principles: 1) the principle of proportionality – whereby an attack that would cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated, is prohibited (See Rule 14 of the Study on customary international law by the ICRC) – and 2) the principle of precaution in attack – which states that constant care must be taken to spare the civilian population, civilians and civilian objects. All feasible precautions must be taken to avoid, and in any event to minimize, incidental loss of civilian life, injury to civilians and damage to civilian objects (See Rule 15 of the Study on customary international law by the ICRC).