After the German defeat in World War I, there arose in Germany the myth that the nation had lost the war because it had been stabbed in the back on the home front. The villains were several, but they included Jews and bankers and the press, although what was not included was any explanation of how these groups benefited from the defeat. It was asking too much. Myths do not need to be logical.

Now, somewhat the same thing is happening here about Vietnam. In no time at all, we have created an entire revisionist history of our involvement in Southeast Asia. It holds that the North started it, that it was not a civil war, that it was worth fighting for America and that it was lost in good measure because it was sold out at home by the press, intellectuals, media figures, weak politicians and all others who are soft on both logic and communism.

There is a trace of this in Ronald Reagan. It seeps out from time to time, most notably in press conferences where he makes what are called "mistakes" in answering questions. Most recently, for instance, he did it when trying to answer a question about El Salvador by citing a bollixed history of Vietnam.

The president said that President Ho Chi Minh of North Vietnam had refused to participate in the elections scheduled for 1956 by the Geneva agreement that ended French control over Vietnam. In fact, it was the leader of South Vietnam, Ngo Dinh Diem, who ducked the elections--and for very good reasons; he feared a victory by the North -- and so, for that matter, did the American secretary of state, John Foster Dulles.

The president also was wrong about when and how the first American combat troops got to Vietnam. John Kennedy did not send Marines, as Reagan said. Lyndon Johnson did -- first 500 and then 3,500 more, and then more and more and, of course, many, many more.

Reagan's version of events was called a mistake and indeed it was. But it was not the sort of mistake that could be corrected by either data or information. It was a mistake that seemed to come from the heart, that stemmed from an almost emotional need to believe that the Vietnam War started differently than it did and that we were there for different reasons than we were.

The fact is that Reagan's version of history corresponds neatly with his other statements about Vietnam. For instance, in his address to the Veterans of Foreign Wars convention on Aug. 18, 1980, he called the Vietnam War "a noble cause," and then explained what he meant:

"A small country newly freed from colonial rule sought our help in establishing self-rule and the means of self-defense against a totalitarian neighbor bent on conquest," Reagan said. And then he went on to say that a lesson of the war was that "if we are forced to fight, we must have the means and the determination to prevail or we will not have what it takes to secure the peace." There is no mention here of the corrupt Saigon regime, of the indigenous rebellion in the South, of the fact that the people in the North were also Vietnamese or, for that matter, of the legitimate role that American public opinion played in bringing the war to an end.

It ought to be clear by now that there are two components to the Reagan view of what happened in Vietnam. The first is that a struggling, independent and altogether worthy nation was attacked by a vicious neighbor, and the second is that the American military could have won the war if it were not sold out at home. These are not simple historical "mistakes." They are, instead, manifestations of a need to believe something, a need to account for what turned out to be such an unpleasant national experience. Someone has characterized this as the 19th hole history of Vietnam. But call it what you want, it is widespread and deeply emotional and could in time become a real national myth--like the old one about how happy the slaves were on the plantation.

The trouble is that curled up in the Vietnam myth lurks the hint of a next time--a chance to do the thing all over again but to do it right. Reagan, in fact, recited his history of Vietnam in response to a question about whether El Salvador could be Vietnam all over again. History will be the final judge of that, but for the moment his answer leaves open one possibility: Not unless he wants it to be.