Congress defines a prisoner of war as a person who, while serving on active duty, was forcibly detained by an enemy government or a hostile force, during a period of war or in situations comparable to war.
At the end of the Vietnam War, there were 2,585 unaccounted for American prisoners, missing in action or killed in action/body not recovered.
As of August, 2002, 1,905 Americans are still missing and unaccounted for.
Vietnam War
The Tunnels of Cu Chi
Paralumun New Age Village