Distinction (the principle of distinction) is a principle under international humanitarian law governing the legal use of force in an armed conflict, whereby belligerents are required to distinguish between combatants and non-combatants (protected civilians): parties to a conflict must “direct their operations only against military objectives.” Combatant in this instance means persons entitled to directly participate in hostilities and thus are not afforded immunity from being directly targeted in situations of armed conflict. Protected civilian in this instance means civilians who are enemy nationals or neutral citizens outside of the territory of a belligerent power. Distinction, proportionality, and precaution are important factors in assessing military necessity in that the harm caused to protected civilians or civilian property must be proportional and not "excessive in relation to the concrete and direct military advantage anticipated" by an attack on a military objective.

The principle of distinction is a fundamental axiom of the modern law of war—also known as the law of armed conflict or international humanitarian law (IHL)—and requires that only military objects may be lawfully targeted while non-military objects, such as civilian objects, remain immune. Regarding persons, only combatants may be lawfully targeted unless rendered hors de combat by illness, injury, or capture. In order to comply with the principle of distinction, parties to an armed conflict must identify combatants and military objects from non-combatants and non-military objects, such as civilians and civilian objects, and combatants rendered hors de combat due to injury, illness, or capture. This means that direct attacks may only be launched against military objectives, while non-military objects (such as civilian objects) are immune from attack. The principle of distinction is applicable in both international and non-international armed conflicts.

"Sudan, Rwanda, the Former Yugoslavia, Northern Ireland, Israel-Palestine, and the Nagoro-Karabakh region of Azerbaijan are all places where the Principle of Distinction has been systematically violated in recent conflicts."

In interstate or international armed conflicts (IAC), the requirement of distinction between combatants and protected civilians lies at the root of the jus in bello. The principle of distinction is reflected in the treaty law of armed conflict, such as in Article 48 of the 1977 Additional Protocol I to the Geneva Conventions of 1949 for the Protection of War Victims: "the Parties to the conflict shall at all times distinguish between the civilian population and combatants and between civilian objects and military objectives and accordingly direct their operations only against military objectives."

In non-interstate or non-international armed conflicts (NIAC), no requirement of distinction exists under Additional Protocol II to the 1949 Geneva Conventions. However, it did state under Article 13 of Additional Protocol II that civilians "shall enjoy general protection against the dangers arising from military operations" until "they take a direct part in hostilities."

The principle of distinction is set forth in Article 48 of Additional Protocol I: "the Parties to the conflict shall at all times distinguish between the civilian population and combatants and between civilian objects and military objectives and accordingly shall direct their operations only against military objectives." Other provisions of Additional Protocol I further outline this requirement of distinction, including in Articles 50, 51, and 51(3). Article 50 of Protocol I defines who is a civilian and what is a civilian population; Article 51 describes the protection which should be given to civilian populations; Chapter III regulates the protection of civilian objects. Article 51(3) of Protocol I states that "civilians shall enjoy the protection afforded by this section, unless and for such time as they take a direct part in hostilities."

Article 8(2)(b)(i) of the Rome Statute of the International Criminal Court (ICC) also prohibits attack directed against civilians. Not all states have ratified Protocol I or the Rome Statute, but it is an accepted principle of international humanitarian law (IHL) that the direct targeting of civilians is a breach of the customary laws of war and is binding on all belligerents.

Luis Moreno-Ocampo was the Chief Prosecutor of the International Criminal Court who investigated allegations of war crimes during the 2003 invasion of Iraq. He published an open letter containing his findings, in a section titled "Allegations concerning War Crimes," he elucidates this use of distinction: