Vietnam ended America's innocence in international affairs. It was the first war in which involvement was not triggered by overt aggression of organized units across a clearly demarcated line. It was the first in which some sort of military outcome did not precede negotiations. It was the first witnessed in the living rooms of America. It was the first in which prominent Americans opposed their country's policy during highly publicized visits to the enemy's capital.

In the process, Vietnam turned into a tragedy in four acts.

Act 1: The Flawed Assumption. In his inaugural address President Kennedy announced that the United States would "support any friend, oppose any foe to assure the survival and success of liberty." No one challenged that sweeping commitment or the proposition that Indochina was a key outpost in the defense of liberty. Within six weeks Marines were sent to Thailand; a year later 16,000 U.S. military personnel were assigned as "advisers" to help South Vietnam. Hanoi was regarded as the cutting edge of Sino- Soviet global strategy. In retrospect, we know that Hanoi was working for its own account.

The ultimate political goal was noble: to enable a distant people to resist tyranny. On the other hand, the so-called free countries of Indochina, while far less oppressive than North Vietnam, were hardly democracies. Guerrilla wars are rarely pristine. The pace of guerrilla war and the pace of reform are different: bringing about democracy in a developing country requires a decade or more; destruction and chaos can be produced in weeks.

Refusal to face this reality caused the Kennedy administration to encourage -- to put it mildly -- the overthrow of South Vietnam's authoritarian ruler Ngo Dinh Diem in 1963. This was the watershed leading to two fateful decisions: it committed the United States to sustain the junta that replaced Diem and it tempted Hanoi to commit its regular forces.

Act 2: The Ambivalent Strategy. America historically has sought to use its vast resources for a strategy of attrition; attrition, however, cannot work against guerrillas who defend no territory and are able to choose their own time for combat. In Indochina, moreover, they were operating from sanctuaries in all neighboring countries and were fought by the fashionable theory of gradual escalation designed to create pauses that would encourage compromise. In fact, gradual escalation convinced Hanoi that America lacked resolve.

As the war dragged on, demands for a political solution mounted. But they were bedeviled by the traditional American tendency to treat power and diplomacy as separate. It became a commonplace that North Vietnam would not negotiate -- indeed could not be asked to negotiate -- while its territory was being bombed, never mind the North Vietnamese troops illegally invading Laos, Cambodia and South Vietnam. President Johnson finally overcame his instinctive doubt and agreed to a bombing halt shortly before the 1968 election. In Korea the decision to end offensive operations after negotiations had started was responsible for 60 percent of U.S. casualties; in Vietnam the bombing halt -- which I supported at the time -- surely deepened the stalemate.

In the process bipartisan support for foreign policy evaporated. Between 1963 and the end of 1966, media, public and congressional support had been nearly universal. But by late 1966 the war became a rallying point for heretofore fringe groups seeking a radical transformation of society.

Too often, the media became unwitting collaborators. It was easy to record the horrors of modern warfare, much more difficult to distinguish between what was inherent in modern weaponry and what represented deliberate cruelty. Similarly, it was fairly simple to construct the vaunted credibility gap by reiterating the difference between governmental statements and what in fact happened. A fairer analysis would have sought to determine what was due to genuine confusion and what was actual misrepresentation.

Act 3. The Painful Exodus. No one familiar with Richard Nixon's career could have believed that his campaign promise to end the war could mean simple abdication. On the contrary, it was surprising that a president elected by a conservative constituency went to such lengths to placate liberal critics. But in the prevailing atmosphere of radicalization, every concession elicited further demands, culminating in pressures to withdraw unilaterally and overthrow the government of America's ally.

Nixon was convinced that it was immoral and dangerous for Americato extricate itself by simply abandoning millions who had fought with it in reliance on its word. He undertook to salvage America's honor as he saw it by a tour de force: phased troop withdrawals to placate the protesters, private negotiations, sporadic pressures on North Vietnam and major assistance to South Vietnam. Domestic pressures forced Nixon into compromises that often canceled themselves out. Every withdrawal encouraged Hanoi and every lunge inflamed the peace movement.

In the end, a president cannot conduct a war amid such passions by himself. Faced with congressional resolutions that progressively edged toward unilateral withdrawal, violent demonstrations and the hostility of the media, Nixon should have gone to Congress early in his term, outlined his strategy and demanded an endorsement. Failing that, he should have liquidated the war. He rejected such advice because he felt history would never forgive the appalling consequences of what he considered an abdication of executive responsibility. It was an honorable, highly moral decision.

Despite all obstacles Nixon came heartbreakingly close to success. By the end of 1972, his administration had forced Hanoi to accept two irreducible conditions: America would not end the war by overthrowing an allied government, nor would it forgo the right to assist peoples that had fought valiantly at its side. What destroyed these prospects was the collapse of executive authority because of Watergate after signing of the 1973 Paris accords.

Act 4: The Post-1973 Period. The apostles of America's inherent iniquity have propagated the canard that all the Nixon administration sought was a fig leaf for South Vietnam's inevitable collapse. This is untrue and unworthy. To be sure, there were terms that one would have preferred to improve, but the Nixon administration believed it had achieved an acceptable settlement -- all the more so as the alternative was a congressional cutoff of funds leading to a total collapse. We were not naive about Hanoi's goals but we saw several elements of enforcement: continuing aid to enable the South Vietnamese army to handle low-level violations; the threat of American retaliation against massive, cross-border violations; the restraining influence of Moscow and Peking, which had growing stakes in their relations with the United States; and an offer of American aid to Hanoi if it chose to rebuild the north instead of conquering the south.

But the peace accords did not end the fevered Vietnam debate, now reinforced by Watergate. The rewards and penalties so painfully assembled were systematically dismantled. Despite immediate and flagrant North Vietnamese violations, Congress voted in June 1973 to prohibit any American military action "in, over or near" Indochina. It cut appropriations to Vietnam by 30 percent in 1973 and by another 50 percent in 1974. It put a paltry ceiling on aid to Cambodia, prohibiting any American advisers and even the transfer of American equipment from nearby Asian allies. It launched an assault on d,etente at a time of maximum weakness of the executive branch.

President Nguyen Van Thieu panicked when it became clear he would not receive the supplementary appropriation he had been promised for 1975. And Hanoi decided to throw the dice after having occupied a provincial capital, demonstrating that not even the grossest violation would be met by American retaliation.

We shall never know whether South Vietnam could have held out with a more generous and resolute American policy. But that is not the point. The United States owed the peoples of Indochina a decent opportunity for survival; its domestic divisions made it impossible for the United States to pay this debt.

What is one to learn from this sequence of events?

* Guerrilla wars are best avoided by pre- emption, by generous programs of assistance and reform in countries the United States considers vital. But once a war is in progress, victory cannot be achieved by reform alone.

* Before America commits combat troops it should have a clear understanding of the nature of the threat and of realistic objectives.

* When America commits itself to military action, there is no alternative to achieving the stated objective.

* A democracy cannot conduct a serious foreign policy if the contending factions do not exercise some restraint in their debate.

If Vietnam is to leave any useful legacy, America owes it to itself to make a fair assessment of the lessons of that tragedy. That has not yet occurred.

Radical critics seek to impose a version of history according to which bloodthirsty leaders sustained a war with no purpose except to satisfy twisted psychologies. The right distorts history by simply ignoring Vietnam. Its isolationist wing had always been more comfortable with strident anti-communist rhetoric than with commitments to fight communism on distant battle fronts.

The lapse of a decade should enable America to face its past. As it turned out the dominoes fell visibly only in Indochina. But the experience of Vietnam is deeply imprinted in the intangibles by which other nations judge America's staying power and even more in the willingness of America to defend its vital interests or even to define them. On the other hand, the Soviet Union after a spurt of expansionism is mired in contradictions. Vietnam, by its singleminded brutality, has turned itself into a pariah.

America failed in Vietnam, but it gave the other nations of Southeast Asia time to deal with their own insurrections. And America's very anguish testified to its moral scruples. Once again, free peoples everywhere look to America for safety and progress. Their greatest fear is not America's involvement in the world but its withdrawal from it. This is why 10 years after the sadness of Saign's fall American unity is both its duty and the hope for the world.