To most of us, it no doubt seemed no more than a political routine when President Bush opened a speech to a veterans' group on the subject of Iraq by promising that he would not forget American POWs and MIAs in Southeast Asia. He might well have thought that with thousands of Americans -- civilian hostages and military personnel -- now at risk in the Persian Gulf, it was a fitting moment to recall the military men who are still unaccounted for from an earlier war.

To a small band of Americans, however, what was going on here was likely to be seen not as a familiar political routine but a monstrous political conspiracy. They believe it is being spun out not by Communists hiding away Americans for their later bargaining purposes, which is what many of us might suspect from Vietnam. Rather, they think, it is the work of American officials, military and civilian, at the policy and working levels, who have neglected their supreme obligation to keep faith with fellow Americans sent to war.

These people reject the judgment of successive presidents that, in Bush's customary formulation, there is no conclusive evidence that live Americans remain behind, but there is an assumption that some may and so the United States must keep trying to find and retrieve them.

To this minority, there is such evidence, arising mostly from refugee sightings, and it is locked away in the classified files by officials with devious venal, bureaucratic or political considerations on their minds. Some go on to suggest that there is also evidence of prisoners left over from the Korean War and even from World War II -- a pattern of Communist treachery.

No one who has not seen the files would want to swear that nothing in them supports at least a distant hope of recovering live prisoners. But from talking with those who know the files intimately and from sampling the conspiracy materials, it is possible to form a view.

Fifteen years after the American failure in Indochina, hundreds and perhaps thousands of officials from the White House, intelligence agencies, Pentagon and State Department and others have studied these files and have had an opportunity to act on or leak their contents, and nothing indicating an official coverup has persuaded more than the believers. The head of the non-governmental National League of Families of American Prisoners and Missing in Southeast Asia has a seat at the interagency table.

Presidents Nixon, Ford and Carter, pressing on after North Vietnam returned the acknowledged prisoners, came up empty-handed. Ronald Reagan went into high gear, accepting a broader possibility that some prisoners had survived, altering and increasing intelligence collection, opening contacts with Hanoi, turning up the public visibility and the political heat. All this, of course, was inherently inconsistent with the notion of a coverup. From it came a focus on ''clear discrepancy cases,'' where men were captured alive and then not heard from, and the resolution of some 200 cases in which the remains of bodies were returned.

The American government continues to negotiate on other discrepancy cases on the theory that the answers lie either in Hanoi's archives or in its hands. The official position is that American satisfaction on the MIAs is an essential condition of the normalization of relations with Vietnam -- which proceeds slowly and stickily.

Some suggest that Vietnam may still be holding American POWs against delivery on Nixon's pledge of billions in postwar aid. But it was a conditional pledge, whose conditions Hanoi long ago violated. Anyway, in the ongoing MIA talks, the Vietnamese do not make that link. Notwithstanding the rare turncoat or wanderer, they have not hinted that they hold prisoners. They deny it. Some Americans wonder if accusations that lead them to reiterate denials may not make it harder for them to change course if they ever decided to.

The irony is that the Reagan-Bush emphasis on MIAs itself perhaps helped fuel the idea of a coverup. When no live prisoners were found, some Americans, acting out of an intense combination of loyalty and frustration, refused to make even a symbolic abandonment of their comrades in arms and turned on their own government instead.

My own sense of things is that the Americans who were taken prisoner in Vietnam have been honorably served by those charged with pursuing their fate. Does that make me part of a conspiracy?