(pow, or Pw), any person captured or interned by a belligerent power during war. In the strictest sense it is applied only to members of regularly organized armed forces, but by broader definition it has also included guerrillas, civilians who take up arms against an enemy openly, or noncombatants associated with a military force.

In the early history of warfare (war) there was no recognition of a status of prisoner of war, for the defeated enemy was either killed or enslaved by the victor. The women, children, and elders of the defeated tribe or nation were frequently disposed of in similar fashion. The captive, whether or not an active belligerent, was completely at the mercy of his captor, and if the prisoner survived the battlefield, his existence was dependent upon such factors as the availability of food and his usefulness to his captor. If permitted to live, the prisoner was considered by his captor to be merely a piece of movable property, a chattel. During religious wars, it was generally considered a virtue to put nonbelievers to death, but in the time of the campaigns of Julius Caesar a captive could, under certain circumstances, become a freedman within the Roman Empire.

As warfare changed, so did the treatment afforded captives and members of defeated nations or tribes. Enslavement of enemy soldiers in Europe declined during the Middle Ages, but ransoming was widely practiced and continued even as late as the 17th century. Civilians in the defeated community were only infrequently taken prisoner, for as captives they were sometimes a burden upon the victor. Further, as they were not combatants it was considered neither just nor necessary to take them prisoner. The development of the use of the mercenary soldier also tended to create a slightly more tolerant climate for a prisoner, for the victor in one battle knew that he might be the vanquished in the next.

In the 16th and early 17th centuries some European political and legal philosophers expressed their thoughts about the amelioration of the effects of capture upon prisoners. The most famous of these, Hugo Grotius (Grotius, Hugo), stated in his De jure belli ac pacis (1625; On the Law of War and Peace) that victors had the right to enslave their enemies, but he advocated exchange and ransom instead. The idea was generally taking hold that in war no destruction of life or property beyond that necessary to decide the conflict was sanctioned. The Treaty of Westphalia (Westphalia, Peace of) (1648), which released prisoners without ransom, is generally taken as marking the end of the era of widespread enslavement of prisoners of war.

In the 18th century a new attitude of morality in the law of nations, or international law, had a profound effect upon the problem of prisoners of war. The French political philosopher Montesquieu in his L'Esprit des lois (1748; The Spirit of Laws) wrote that the only right in war that the captor had over a prisoner was to prevent him from doing harm. The captive was no longer to be treated as a piece of property to be disposed of at the whim of the victor but was merely to be removed from the fight. Other writers, such as Jean-Jacques Rousseau and Emerich de Vattel, expanded on the same theme and developed what might be called the quarantine theory for the disposition of prisoners. From this point on the treatment of prisoners generally improved.

By the mid-19th century it was clear that a definite body of principles for the treatment of war prisoners was being generally recognized in the Western world. But observance of the principles in the American Civil War (1861–65) and in the Franco-German War (1870–71) left much to be desired, and numerous attempts were made in the latter half of the century to improve the lot of wounded soldiers and of prisoners. In 1874 a conference at Brussels prepared a declaration relative to prisoners of war, but it was not ratified. In 1899 and again in 1907 international conferences at The Hague drew up rules of conduct that gained some recognition in international law. During World War I, however, when POWs were numbered in the millions, there were many charges on both sides that the rules were not being faithfully observed. Soon after the war the nations of the world gathered at Geneva (Geneva Conventions) to devise the Convention of 1929, which before the outbreak of World War II was ratified by France, Germany, Great Britain, the United States, and many other nations, but not by Japan or the Soviet Union.

During World War II millions of persons were taken prisoner under widely varying circumstances and experienced treatment that ranged from excellent to barbaric. The United States and Great Britain generally maintained the standards set by the Hague and Geneva conventions in their treatment of Axis POWs. Germany treated its British, French, and American prisoners comparatively well but treated Soviet, Polish, and other Slavic POWs with genocidal severity. Of about 5,700,000 Red Army soldiers captured by the Germans, only about 2,000,000 survived the war; more than 2,000,000 of the 3,800,000 Soviet troops captured during the German invasion in 1941 were simply allowed to starve to death. The Soviets replied in kind and consigned hundreds of thousands of German POWs to the labour camps of the Gulag, where most of them died. The Japanese treated their British, American, and Australian POWs harshly, and only about 60 percent of these POWs survived the war. After the war, international war crimes trials were held in Germany and Japan, based on the concept that acts committed in violation of the fundamental principles of the laws of war were punishable as war crimes.

Soon after the end of World War II the Geneva Convention of 1929 was revised and set forth in the Geneva Convention of 1949. It continued the concept expressed earlier that prisoners were to be removed from the combat zone and be humanely treated without loss of citizenship. The convention of 1949 broadened the term prisoner of war to include not only members of the regular armed forces who have fallen into the power of the enemy but also the militia, the volunteers, the irregulars and members of resistance movements if they form a part of the armed forces, and persons who accompany the armed forces without actually being members, such as war correspondents, civilian supply contractors, and members of labour service units. The protections given prisoners of war under the Geneva Convention remain with them throughout their captivity and cannot be taken from them by the captor or given up by the prisoners themselves. During the conflict prisoners might be repatriated or delivered to a neutral nation for custody. At the end of hostilities all prisoners are to be released and repatriated without delay, except those held for trial or serving sentences imposed by judicial processes. See also Geneva Convention (Geneva Conventions).