They were the focal point of 1980s action thrillers such as Sylvester Stallone's Rambo and Missing In Action, starring Chuck Norris.
Now the spectre of American prisoners of war left behind in Vietnam is back, not as Hollywood fiction but as political reality as US President Barack Obama makes his first visit to a nation where the US tried and failed to defeat the advance of communism.
Obama arrives in the capital Hanoi tomorrow to meet Vietnamese leaders in the latest phase of a long rapprochement between the former enemies.
Trade and security issues will be at the forefront of the agenda. Vietnam is pushing for the US to lift its arms embargo as Washington and Hanoi find themselves allies against Beijing's aggression in the South China Sea.
Pressure groups are also urging Obama to speak in public and private about the dismal human rights record of the repressive one-party state.
For some Americans, there is another piece of "unfinished business" that should take centre-stage - the fates of more than 1600 servicemen who never returned from the first lost war in US history. Relatives want the President to demand Vietnam's help in accounting for missing personnel who may have been captured and killed after being shot down during air raids or died during captivity.
In an illustration of how raw the issue remains, the black flag for those known collectively as the MIA/POWs (missing in action and prisoners of war) of Vietnam still flies over the Capitol in Washington.
Some 1026 Americans have been accounted for - and in many cases, had their remains returned - in joint retrieval operations between teams from the two countries that has been part of a drive to normalise bilateral ties.
But some campaigners want Obama to go further by pressing Vietnam to answer accusations that it kept some US prisoners after 1973, rather than releasing all captives as the peace agreement demanded and as both countries have officially claimed.
Vietnam has consistently insisted it provided full assistance to the US about its missing personnel and denied that it held prisoners of war after the conflict ended.
The last of 591 American POWs acknowledged to be in the custody of what was then communist North Vietnam - which was engaged in a long war against US-backed South Vietnam to unify the country - arrived back on home soil in April 1973 after a series of military transport flights called Operation Homecoming.
Suspicions have endured, however, that some detainees were retained by the North Vietnamese as leverage to force President Richard Nixon to deliver a US$3.25 billion aid package he is said to have promised in secret as an inducement to end the war.
The money failed to materialise after the Nixon presidency disintegrated following the Watergate scandal, angering Hanoi, which was supposedly left with American inmates who had been devalued as bargaining chips, the theory goes.
No conclusive proof has been provided to support the thesis, which has been ridiculed as a myth and a conspiracy theory in many quarters.
Jeffery Donahue, 69, has spent nearly half a century campaigning for news of his older brother, Lieutenant Morgan Donahue, whose plane was shot down over neighbouring Laos in December 1968 in an area then under the control of North Vietnamese forces.
"He has never been accounted for and nobody knows what happened to him," Donahue said. "The Government says he is dead. But I'm convinced he was still alive at the end of the war. I don't hold any hope that he is alive now.
He has never been accounted for and nobody knows what happened to him
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"Like so many others, I think he was taken to North Vietnam and eliminated when the communists got angry that they didn't get their US$3 billion and didn't want to become embarrassed by continuing to hold POWs."
Obama, he said, should resolve the issue once and for all by agreeing with Vietnam to form truth and reconciliation committees like those established in South Africa after apartheid.
John McCain, the Republican senator who was a Vietnam prisoner of war, played a key role in investigating the so-called "live prisoner" theories as a member of the chamber's POW/MIA Committee. Alongside John Kerry, a Democrat and fellow Vietnam veteran who as Obama's Secretary of State will join the trip to Hanoi, he said he received "full access" to Vietnamese records in the 1990s and did not believe there were any surviving prisoners.
McCain was vilified as a traitor by some activists for his stance.
But Mark Sauter, an author of several books on missing POWs and a consultant to the National Alliance of Families for the Return of American Missing Servicemen, rejected the criticisms.
"This is not mythology or some kind of Rambo-type thing," he said. "There are still critical issues that have not been answered and we hope President Obama honours the POW-MIAs by using this occasion to ask some questions."