In armed conflicts combatants and civilians are killed, some by mistake, or as collateral damage; some by friendly fire, or mistaken distinction determinations. Most are killed because they were specifically targeted by a member of an opposing armed force, armed opposition group, or revolutionary band.
What is the applicable law of war for such killings? When may human beings, combatants, enemy fighters, or civilians lawfully be killed and when may they not?
Two Predicate Requirements
For a use of force to fall under LOAC/IHL it must, first, take place in the course of an armed conflict. Second, the use of force must have a nexus with the armed conflict. The core principles of distinction, military necessity, unnecessary suffering, and proportionality remain applicable in any application of military force.
Imagine widespread rioting and looting in a Blueland city, with units of Blueland's armed forces called in to maintain order. Civilian looters are captured and, in some cases, shot and killed by Blueland soldiers. In such a scenario LOAC/IHL is not implicated because there is no armed conflict involving an opposing armed group or unit. Instead, customary international law, domestic law, and human rights law would be applicable. Allegations of unlawful use of force by Blueland soldiers would be tried either by court-martial or by domestic courts.
Now consider a Blueland military unit allied with Redland in Redland's noninternational armed conflict against a nonstate armed opposition group inside Redland. Wellarmed narco-criminals have kidnapped the Redland president's family. The Redland president entreats Blueland to intercept the narco-criminals as they cross through Blueland's tactical area of responsibility. Blueland does so and a firefight ensues, with casualties among Blueland troops and the narcos. The first family is rescued. In this case, unlikely in reality as it is, the fighting takes place while an armed conflict is in progress but it has no nexus with the armed conflict. Domestic law and human rights law prevail.
Lawful Combatant Human Targets
Recall that there are only two individual statuses on the battlefield: civilians and combatants. There are numerous subcategories of both. Subcategories of combatant, for example, include prisoner of war, detainee, retained person, and spy.