War, Laws of, in international law, rules and principles regulating an armed conflict between nations. These laws are designed to minimize the destruction of life and property, to proscribe cruel treatment of noncombatants and prisoners of war, and to establish conditions under which the belligerents may consult with one another. To mitigate the effects of insurrections and civil wars, established governments often recognize the belligerency of domestic opponents and conduct conflicts with them according to the laws of war.

In the Middle Ages the ideals of knighthood restrained some cruelties in warfare, but systematic legal codes did not appear until the 17th cent. The great work of Grotius, De jure belli ac pacis [on the laws of war and peace] (1625) and the works of Vattel had much influence in introducing humane practices. Detailed international treaties governing war are mostly a product of the 19th and 20th cent. The Declaration of Paris, the accords concluded at the Hague Conferences (1899, 1907), and the Geneva Conventions (1864, 1906, 1929, 1949) are the main bodies of formulated law.

There is no convention on the laws of war to which all the major powers of the world have acceded, and many conventions provide that their terms shall be inoperative if any of the belligerents is not a signatory or if an enemy commits a violation. Despite such provisions, many nations have adopted the laws of war, and the conditions of warfare have undoubtedly been ameliorated, particularly in the treatment of prisoners and the consideration shown to the sick and wounded. The care of the sick and the wounded is facilitated by making medical personnel noncombatants and by clearly marking hospitals and similar installations, thus sparing them from attack. Conventions restricting the use of certain weapons probably have not materially mitigated the horrors of war. For the most part, only those weapons that are of limited military use, e.g., poison gas, have been effectively banned, while efforts to prohibit militarily effective weapons, e.g., atomic weapons and submarine mines, have not succeeded.

The laws of war have had as their objective the protection of civilian populations by limiting all action to the military. A distinction was made between combatants and noncombatants, the former being defined in terms of traditional military units. Thus combatants must have a commander responsible for subordinates, wear a fixed and recognizable emblem, carry arms openly, and follow the laws of war. But the development of aerial bombing in World War I and of guerrilla forces dependent on civilians has tended to make all enemy territory part of the theater of operations. New practices and categories have yet to be worked out to protect civilian centers adequately.

Civilians in territory occupied by the enemy are, however, supposed to be entitled to certain protections. There may not be imprisonment without cause, and fines may not be levied upon a whole civilian population for individual offenses. Private property also receives limited protection, and it may not be confiscated for military use unless fair compensation is paid. Special rules govern such actions against property as the taking of a prize at sea or in port, the confiscation of contraband, and the use of the blockade. Property destroyed in the course of action against the enemy is, of course, not compensable. Places of religious, artistic, or historical importance should not be attacked unless there is military need.

No direct diplomatic relations exist between belligerents, but neutral diplomats are often given custody of property in enemy territory and are entrusted with negotiations. In the field of combat, passports, safe-conducts, and flags of truce permit consultations between opposing commanders. Hostilities may even be totally suspended by an armistice, which is often the prelude to surrender.

Violations of the laws of war have probably occurred in all major conflicts; a nation confident of victory will frequently not be deterred even by fear of reprisals. After World War II the military and civilian leaders of the Axis Powers who were responsible for violations were tried for war crimes, and some Americans were tried for war crimes in the Vietnam War.