A POW or Prisoner of War is an enemy combatant captured and held during wartime. Very specific rules govern who exactly is defined as a POW, and how POWs are to be treated by their captors. For example, governments must notify each other when they capture POWs. Many governments maintain a POW/MIA office for families of service members who have been captured or who have gone missing in wartime.
Humans have been making war for thousands of years, but the concept of prisoners of war is actually fairly recent. For the bulk of human history, enemy combatants were either slaughtered on the battlefield by the victors, or taken and enslaved for use as a source of cheap labor. Sometimes, former enemies were integrated into the society of the winners, especially if they had valuable skills, but they were typically still treated as second-class citizens.
By the 1600s, the concept of taking prisoners of war and ransoming them to their home governments was widespread enough that there were calls for the unilateral release of prisoners of war without ransom after the end of conflicts. Governments began to realize the political potential of prisoners of war, recognizing that they could be used in a variety of ways, and in 1907, the Hague Convention set out formal definitions for POWs, with refinements being added in 1929 during the Third Geneva Convention.
Under international law, a POW must be a lawful combatant, wearing military uniform, and he or she may surrender or be taken, although some militaries explicitly forbid surrender to enemy troops. This excludes people such as armed militants, terrorists, and guerrillas from the protections offered to prisoners of war. Once captured, the prisoner must not be abused, and the home government must be notified. Prisoners may be released by arrangement in wartime, or held until the end of the conflict and released without conditions.
Some notable people in history were prisoners of war, such as George Washington, Winston Churchill, and Jean-Paul Sartre. Others such as Kurt Vonnegut wrote about their experiences as prisoners of war, and some have become outspoken advocates for POWs and people missing in action (MIA) in military conflicts. In governments with POW/MIA offices, officials routinely work to track down missing prisoners of war and servicemembers, sending investigators to every corner of the Earth to follow up on clues.