This question needs to be asked again following reports this week about a Russian archive document suggesting that 1,205 American prisoners of war were being held in North Vietnamese prisons in 1972. This is double the number of American prisoners subsequently released by Vietnam, and has prompted speculation that Vietnam concealed the real number of Americans held and may still be holding some of them.
As expected, activists from American organisations campaigning for the return of the MIAs they steadfastly believe to exist have seized on the reports as evidence of the rightness of their cause. As expected, relatives and friends of missing servicemen have had their hopes cruelly raised that their loved ones may return. No matter that Vietnamese officials have called the document a 'clear fabrication', or that American officials have counselled caution about its authenticity. One of the admirable American qualities - much envied, incidentally, by Russians - is an inexhaustible capacity for hope.
The authenticity or otherwise of the document, discovered in January by a Harvard researcher in the archives of the Soviet Communist Party's Central Committee in Moscow, will provide only a secondary answer to the question 'why?' Discovery of a document in a Soviet or Russian archive is no guarantee of its authenticity. Access to archive documents is highly selective and still restricted, whatever the Russian authorities may say.
Some restrictions are inadvertent, resulting from nonexistent indexing and general muddle; others are entirely deliberate and reflect decisions taken by the Russian authorities, certain 'organs of state', or even individual archivists.
A file may be provided or refused at the apparent whim of an administrator; some are offered for sale. All manner of considerations apply, according to whether the applicant is deemed to have clout, money, an engaging manner or a pretty face. Nor are foreign researchers usually permitted to view a complete set of documents. Records in the intelligence archives in particular, but now often in the Communist Party archives as well, are mostly pre-selected by trusted Russian researchers. And if documents can be removed before a file is released, they can doubtless be added as well . . .
More productive is to consider why the controversy surrounding America's MIAs should have resurfaced now, and here the precedents are informative. The suggestion that Russia might hold the keys to the secret of America's lost servicemen emerged towards the end of 1991 when the one-time head of Soviet counter-intelligence, the spy-turned-democrat Oleg Kalugin, said during a US lecture tour (not in Russia, note) that Soviet officers had helped to interrogate US prisoners of war in Vietnam. He did not exclude the possibility that some prisoners might have been taken to the Soviet Union. His revelation attracted enormous attention in the US, a fact that would not have escaped Moscow's notice.
In January 1992, the Russian tricolour had scarcely replaced the red flag over the Kremlin dome when the retired KGB officer named by Mr Kalugin as having conducted the interrogation was persuaded to talk about his work in Vietnam. He admitted being present at one interrogation, but said that questioning had been cut short when the American prisoner recognised the Russian accent and objected. The officer was a grudging witness, but his appearance at a Moscow press conference kept the subject alive.
A month later, the possible presence of American prisoners of war in Russia was raised again, this time by the Russian security ministry - the heir of the Soviet KGB, which is still staffed mainly by old KGB officers and works out of its building, the Lubyanka. The ministry held a press conference to advertise its new openness. And what should it have chosen for its star turn but the subject of American prisoners of war?
As well as disclosing that the Soviet Union had shot down US spy planes on at least two occasions before the notorious U2 incident in May 1960, the ministry's officials hinted that American prisoners of war might have been transferred to the Soviet Union during the Vietnam war and could still be alive. The all-consuming interest shown by American reporters in this subject, compared with all the others, would have convinced the Russians that this was still a highly emotive topic and well worth pursuing.
Two months later a Russian researcher offered a further contribution. He claimed to have spoken to an army or intelligence officer who had accompanied an American prisoner being taken from Vietnam to the then Soviet republic of Kazakhstan during the Vietnam war. Neither officer nor prisoner were found or identified, but interest was again kindled in the possibility that missing American servicemen were alive and well, and that Russia could help to find them.
In June, with precision timing, 10 days before Boris Yeltsin's first Washington summit as Russian president, Russia reminded the world that lost Americans were occasionally found - by producing an elderly American defector who had spent the past 40 years in a Russian psychiatric hospital.
The weekend before the summit, Russia drove the point home, publishing a letter sent by Mr Yeltsin to US senators, which gave details of American citizens who, it said, had been held or interrogated in the former Soviet Union. They included 59 US servicemen said to have been interrogated during the Korean war, and deserters from the Vietnam war in transit.
This first post-Soviet summit was a diplomatic risk for Mr Yeltsin, but he was able to make splendid capital from these disclosures. They combined a frankness about the unpleasant past unknown in the former Soviet Union, an understanding of the concern felt by 'ordinary Americans', and a genuine desire to help further. The only awkward moment came when Mr Yeltsin was asked why, if these prisoners existed, none had yet been found. His reply illustrated all the diplomatic potential of the subject: accusing his predecessor, Mikhail Gorbachev, of covering up the facts, he asked: 'Which approach is better, mine or his?'
The MIA 'lever' was used in Washington to brilliant effect. And the glorious thing about it is that this lever can be applied as often as necessary. That summit was followed up with visits to far-flung parts of Russia by a joint US-Russian investigation commission, and delighted local authorities scoured their archives for evidence of American prisoners on their territory. In the latest instance Russia, no doubt, will offer Washington whatever assistance it can to help check the claims contained in its archive document.
The sordid truth is that the MIAs have become an ideal lever in Russia's relations with the US. The Russian security ministry has inherited from its KGB predecessor a fine understanding of the passions raised by this issue, and seems bent on using them to the full. The slightest touch from Moscow will produce a wave of positive sentiment throughout America for a regime which is 'finally coming clean' and could yet help to trace long-lost relatives.
Every time Russia wants to remind Washington of its goodwill, it has only to unearth another shred of evidence about the missing prisoners of war. And why does Moscow need Washington's goodwill now? To reassure Americans that even if it is forced to veto a Western-led motion on intervention in Bosnia, this is for entirely domestic reasons and should not be seen as a slight against its good friend - and potential economic benefactor - President Bill Clinton.