Laws and customs of war

The law of war is a legal term of art that refers to the aspect of public international law concerning acceptable justifications to engage in war (jus ad bellum) and the limits to acceptable wartime conduct (jus in bello or International humanitarian law).

Among other issues, modern laws of war address declarations of war, acceptance of surrender and the treatment of prisoners of war; military necessity, along with distinction and proportionality; and the prohibition of certain weapons that may cause unnecessary suffering.

The law of war is considered distinct from other bodies of law—such as the domestic law of a particular belligerent to a conflict—that may provide additional legal limits to the conduct or justification of war.

Attempts to define and regulate the conduct of individuals, nations, and other agents in war and to mitigate the worst effects of war have a long history. The earliest known instances are found in the Mahabharata and the Torah (Old Testament).

In the Indian subcontinent, the Mahabharata describes a discussion between ruling brothers concerning what constitutes acceptable behavior on a battlefield:

One should not attack chariots with cavalry; chariot warriors should attack chariots. One should not assail someone in distress, neither to scare him nor to defeat him ... War should be waged for the sake of conquest; one should not be enraged toward an enemy who is not trying to kill him.

An example from the Deuteronomy 20:19–20 limits the amount of acceptable collateral and environmental damage:

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