Correction Appended

HISTORY abounds in what-ifs. If Lincoln had not been shot, the South, and the nation, might have accepted full racial equality a century earlier. If Wilson had not been felled by a stroke, the United States might have joined the League of Nations and prevented World War II. And if John F. Kennedy had not been killed in Dallas could the Vietnam War have been avoided?

In ''An Unfinished Life: John F. Kennedy, 1917-1963'' (Little, Brown), the presidential historian Robert Dallek again raises the old question. He argues that Lyndon Johnson misread Kennedy's intentions regarding Vietnam, increasing the American commitment rather than seeking the gradual withdrawal that Kennedy may have intended.

People have powerful reasons for wanting to believe this argument. Not long after Kennedy's death the country went from one crisis to another, stumbling through years of race riots and civil disobedience, the counterculture revolt, the crumbling of authority and the erosion of traditional values.

Above all the war consumed the nation in anger, disillusion and self-doubt. Shock waves generated in the 1960's have never fully abated, and are seen today in the rhetoric of political correctness and neoconservatism.

Mr. Dallek, citing newly available documents and his reappraisal of the existing record, maintains that Kennedy was planning to wind down the Vietnam commitment during his second term, and instructed Defense Secretary Robert S. McNamara to chart a possible withdrawal by 1965. Kennedy, he argues, was becoming skeptical of the hawkish advice he was getting from his military and political advisers. The ill-fated Bay of Pigs landing, which his advisers urged on him, gave good reason for that caution.

At the height of the Cuban missile crisis, it later become known, Kennedy overruled his advisers and accepted a secret deal with the Soviet Union that ended a confrontation that threatened nuclear war. Kennedy even mused in private conversations with journalists about the possibility of lifting the embargo on Cuba in return for Castro's neutrality in the cold war.

If this was Kennedy's intention, why did Johnson continue the impasse with Cuba and substantially expand the involvement in Vietnam? Because, Mr. Dallek maintains, the dead president's chief foreign policy aides, Mr. McNamara, Secretary of State Dean Rusk and the national security adviser, McGeorge Bundy, told him that was what Kennedy wanted. Since Kennedy had never shared any doubts with his vice president, Johnson, a novice at foreign affairs and eager to prove that he was marching in Kennedy's footsteps, did what he was persuaded he must do.

That Kennedy was planning to pull out of Vietnam after his re-election is, Mr. Dallek admits, hotly contested by other historians, an old argument that ''can never fully be put to rest.''

Many will find it persuasive. But the argument will not stop here, or probably ever stop. This is because it rests not only on reading Kennedy's mind, but on taking into account the Kennedy image that has been created over the years and exerts a powerful hold on millions of Americans.

EVER since his assassination in 1963 there have been two John Kennedys. One is the attractive politician who came to the presidency in 1961 promising to ''get the country moving again'' and immediately became embroiled in one foreign crisis after another.

The other John Kennedy is the bigger-than-life, and certainly bigger-than-death, leader who would have made it come out all right, the man who so many Americans (though not most historians) consider to have been one of the greatest presidents.

Mr. Dallek makes a good case. But, like all what-ifs, some of it rests on conjecture. Kennedy had, after all, greatly increased the nation's nuclear arsenal and military budget. He ardently believed in counterinsurgency warfare and believed it vital to demonstrate that Communist-inspired ''wars of national liberation'' could not succeed. He willingly chose Vietnam as the testing ground.

Although in a television interview in September 1963 he said of the South Vietnamese that ''in the final analysis it is their war'' to win or lose, he also added: ''I don't agree with those who say we should withdraw. That would be a mistake.''

As for Cuba, even as Kennedy mused about a guarded reconciliation, plots to kill Castro, under the general direction of Robert Kennedy, continued until they were rescinded by Johnson.

In ''Kennedy's Wars'' (Oxford, 2000), the historian and strategist Lawrence Freedman maintains that despite anecdotal evidence of a desire for a withdrawal from Vietnam after the 1964 elections, it was ''less of a definite decision than a working assumption, based on a hope for stability rather than an expectation of chaos.''

Robert Kennedy seemed to confirm this. When asked in 1964 what his brother would have done if the South Vietnamese had been on the brink of defeat, he replied, ''We'd face that when we came to it.''

While critical of Johnson's expansion of the war, the historian Frederik Logevall in ''Choosing War'' (University of California Press, 1999) cites the Kennedy administration's ''complete rejection of exploring the possibilities for a political solution to the conflict'' when only such a path offered realistic hope for withdrawal. ''On the day of his death,'' Mr. Logevall writes, ''Kennedy had not decided what to do with his Vietnam problem.''

This baleful conclusion gains support from a trusted adviser of Kennedy, Theodore Sorensen, who is quoted in Thomas Brown's ''JFK: History of an Image'' (Indiana University Press, 1988) as saying that Kennedy ''was simply going to weather it out, a nasty, untidy mess to which there was no other acceptable conclusion.''

In the end, what Kennedy wanted to do or might have done is likely to remain a mystery, its secret locked in the recesses of bureaucracy, political ambition, expediency and cowardice, and ultimately in the darkness of the human heart. If Mr. Dallek argues well, so do those on the other side.

All who lived through Vietnam are part of the tragedy, whether we fought, or opposed, or quietly observed the war from a safe distance. Until the 1968 Tet offensive, which shattered predictions of an imminent victory, the vast majority of Americans supported the war.

It is not just Kennedy's or Johnson's but, to use the title of A. J. Langguth's recent study of the war, ''Our Vietnam.''

Photo: Some of the president's men: John F. Kennedy with Defense Secretary Robert S. McNamara, left, and Secretary of State Dean Rusk in December 1962. (Reuters/Bettman)