Captivity is as old as war itself. It has existed for centuries and has profoundly affected the lives of millions of people, including some of the most influential figures in history. Politicians like Winston Churchill and Charles de Gaulle and writers like Miguel de Cervantes and P. G. Wodehouse have all endured imprisonment in wartime. It might even be argued that captivity, which historically has affected entire populations, has been more universal than combat, which usually involves only the warriors of any society. W. Wynne Mason, a historian of New Zealand POWs, has estimated that one person in every thousand was interned at some point during World War II. Even in that global conflict, armed combat occurred in only a few regions. Internment, on the other hand, occurred in all nations, belligerent and neutral, and affected the lives of people thousands of miles from the firing lines.

Any discussion of captivity in wartime must account for the fact that, throughout history, there have been many kinds of prisoners, each of which experienced different treatment. Most notable (and certainly most glamorous in terms of modern culture) have been prisoners of war, called POWs, members of a recognized military body captured during wartime. POW is apparently a straightforward classification that should admit of little controversy, but even within this group, a number of subcategories have evolved. Guerrilla fighters and merchant mariners are, to varying degrees, combatants, but their status as POWs has been the subject of considerable debate. The status of camp followers, civilians who attach themselves to armies to provide services or for personal reasons, has also been unclear. Even legitimate POWs have sometimes found their status called into question. The Allied powers in World War II created other categories to cope with the massive numbers of enemy soldiers captured. Some of them were classed as Disarmed Enemy Forces (DEF) or Surrendered Enemy Personnel (SEP), artificial distinctions that relieved the Allies of according them all the rights of official POWs. By the same token, Italian soldiers captured by the Allies were often classified not as POWs but as military internees, another dubious distinction that owed more to the demands of the moment than anything else. Finally, war captivity has also involved civilians who have been interned for a variety of reasons. While they are not prisoners of war in the strictest sense, they have certainly been prisoners in wartime.

Over the millennia, there have been many changes in captivity as it affected these various groups. Most striking are the improvements in international law, which have created a broad and complex network of largely nongovernmental organizations to monitor and improve the conditions of captivity during wartime. But these improvements make it easy to lose sight of a more sobering reality: that, despite the Enlightenment, notions of progress, and the evolution of a humanitarian spirit, there is far more continuity than change in the institution of war captivity.