By Don Oberdorfer

THE DAY THAT Saigon fell, many Americans and people everywhere believed that the world had changed. Ten years later scars still show and some wounds are still open, but the changes wrought by Vietnam in international relations have been fewer and less clear-cut than had been generally expected.

Since 1945, the United States had been the colossus of military, economic and political power. As the only major industrial nation to emerge from World War II unscathed -- if not even stronger -- America had enjoyed a position of predominance that was rare in human history.

The United States entered the Vietnam war with the breezy self-confidence of a young warrior. It limped away doubting itself and its powers.

The U.S. war effort in Vietnam and its humiliating outcome still resonate powerfully -- as the current anniversary outpouring of articles, broadcasts, books and commemorative events suggest -- but it is less clear that in historical terms its significance will be great.

The overall strategic balance between East and West has been affected by gains and losses for both sides in the past decade. But there is no sign today that the outcome in Indochina a decade ago tipped the scales of geopolitics in any decisive fashion.

Today, the United States is not as headstrong in its global enterprises as it had been before Vietnam. Some important and possibly lasting effects -- for better or worse -- can be seen in the cautious attitudes of U.S. foreign policy makers and the public toward American military involvement abroad. Nonetheless, the United States is still the world leader in almost every area that is a measure of power and influence.

Vietnam, the other nation most directly affected, won the national unification its communist leadership and most of its people had sought, but quickly lost its special place of world esteem and its economic and political prospects. Since 1978, Vietnam's Prussians have been embroiled once again in war, this time to establish and maintain an iron grip on Cambodia, which they often have dominated in the past.

Southeast Asia's supposed "dominoes," Thailand, Malaysia, Singapore and Indonesia, which were thought to be endangered if Vietnam fell, are doing well economically and are relatively stable politically. They all enjoy excellent relations with the United States. The Philippines is in trouble, but for essentially internal reasons.

The Soviet and Chinese backers of the Vietnamese victors have endured the decade in struggle, some of it between themselves. The fault line of the Sino-Soviet split runs like a jagged edge through Indochina, where Moscow and Peking still fuel a proxy war.

What then are the lasting consequences in international relations of the U.S. failure in Vietnam from the perspective of 10 years later?

First, it is important to separate what changed in the world as a result of Vietnam from what was already changing before and would have changed anyway.

Second, it is necessary to examine what effects on the conduct of American foreign policy were wrought by Vietnam, and of these which persist and which have faded.

The French historian and essayist, Raymond Aron, argued in his book, "The Imperial Republic," that Vietnam marked the turning point for the U.S. "empire" in the post-World War II era. "Between ascent and descent, supremacy and humiliation, there lies one outstanding event, the war in Vietnam, started by the French army in 1946 and taken over by the Americans in 1954 immediately after the Geneva agreements," Aron wrote.

In fact, though, the margin of U.S. supremacy had been diminishing for years as Europe, Japan and Russia recovered from the the ravages of World War II and developing countries shook free of their colonial pasts. Even without Vietnam, this country would have found itself by the mid-1970s giving ground to other nations and forces as its unnatural position of dominance was eroded.

According to the International Monetary Fund, the United States accounted for close to 45 percent of the total productive output of the free-market world in the early 1950s, an incredible wealth for a nation with 6 percent of the world's population.

By the mid-1960s, when U.S. involvement in Vietnam was beginning to escalate, the U.S. share of free-market world output had already declined to around 39 percent. By the fall of Saigon in 1975, America's share had plunged to 29 percent. (After a further decline in the past decade, the U.S. share is increasing modestly again to about 33 percent as this nation grows more quickly than others.)

Somewhat the same loss of overwhelming predominance has happened militarily and politically, which is perhaps the most difficult fact of our time for Americans to accept, understand and act upon without either underestimating or exaggerating the U.S. role.

Among the most important legacies of Vietnam is what economists call the "opportunity cost" -- potential gains that were not realized because resources were diverted to a different path.

U.S. military spending on Vietnam in 1968, the military high point of the war, is estimated by the Pentagon to have been $26 billion. This is the equivalent of $86 billion in today's inflated dollars, about two-thirds of the total sum the Pentagon is spending this year for its general-purpose forces stationed in this country and around the world.

The allocation of such large sums to the Vietnam effort, and the temporary decline in U.S. military spending in the immediate aftermath of defeat, cost the United States seven or eight years of military modernization, according to former Undersecretary of Defense Robert W. Komer. An enthusiastic supporter of the Vietnam effort in its early stages and an architect of the pacification program, Komer says today that "in my view the most significant strategic cost of the Vietnam war was the enormous investment for so little result."

While the United States was absorbed by Vietnam, the Soviet Union was extending its strategic nuclear power and the global reach of its conventional forces, closing the military gap with the other superpower. Soviet military parity, which had long been forecast and feared in official Washington, undoubtedly was hastened by the U.S. drift.

Similarly, there was a political "opportunity cost." The tremendous American concentration on Vietnam from the death of Ngo Dinh Diem (and John F. Kennedy) in 1963 to the Paris accords in January 1973, skewed the focus of U.S. diplomacy for a decade. While America was consumed with Vietnam and, under President Nixon, with Soviet and Chinese relationships which were related to Vietnam, explosive forces were building, almost unnoticed by official Washington, in the Middle East, Africa and Central America.

Perhaps the most profound effect of the war, though some prefer to ignore it and others to exaggerate it, was in the heads of American policy makers.

Vietnam drove home the reality that, for all the nuclear weapons in its arsenal, there are limits to what the United States can accomplish beyond its shores. Since the end of World War II the dominant strain in the foreign policy apparatus had assumed that this country could do almost anything it set its mind to do, in pursuit of an important national interest. Until Vietnam.

Some have described Vietnam as the end of innocence, but it may be truer to say it was the end of taking-on-faith, both at home and abroad, that the United States would inevitably succeed in whatever it undertook beyond its shores.

A rereading of documents of the Defense Department's secret history of the war, known as the Pentagon Papers, is startling testimony to the unlimited vistas of the U.S. policymakers of that day.

In January 1964, Gen. Maxwell Taylor, chairman of the Joint Chiefs of Staff, reported to Defense Secretary Robert S. McNamara that, in keeping with a government objective of victory in Vietnam, "we must prepare for whatever level of activity may be required and, being prepared, must then proceed to take actions as necessary to achieve our purposes surely and promptly." The uniformed chiefs recommended a broad program of U.S. and South Vietnamese ground combat, bombing and mining throughout Indochina.

Walt W. Rostow, then chief of State Department policy planning, recommended a similarly broad use of U.S. military power in Indochina as well as the dispatch to the Pacific of "massive forces to deal with any escalatory response, including forces evidently aimed at China as well as North Vietnam."

Rostow wrote Secretary of State Dean Rusk, "Our assets, as I see them, are sufficient to see this thing through if we enter the exercise with adequate determination to succeed." The real margin of influence on the outcome, Rostow wrote, "flows from the simple fact that at this stage of history we are the greatest power in the world -- if we behave like it."

Two decades later, when senior policy makers focused their attention on Central America, the limitations were much more in evidence. When Secretary of State Alexander M. Haig Jr. proposed a U.S. military blockade of Cuba in late 1981 and early 1982, the Joint Chiefs of Staff were influential in rejecting it within the high councils of government. The uniformed chiefs argued, according to a participant in the discussions, that U.S. military forces might not be able to stop a likely Soviet countermove elsewhere in the world, or do so without a national mobilization that the public or Congress might not support.

A State Department internal policy paper on Central America prepared for incoming Secretary of State George P. Shultz in June 1982 described objectives, costs and problems in terms dramatically different from those in the 1960s: "Assuming that Cuba and Nicaragua do not substantially increase the stakes in Central America, the secret to success will be a steady and sustained effort . . . . The use of U.S. combat forces would be counterproductive (and unacceptable here). Rather, we must continue the steady improvement in the military capabilities of friendly nations, reconstruction of their economies, pursuit of social and economic reforms and development of democratic institutions. We must also keep pressure on Cuba and Nicaragua.

"To sustain our effort we will need a steady flow of security and economic resources at an annual rate of about $750 million for each of the next three years . . . . Our principal difficulty in pursuing this policy will be to obtain the required congressional support. Human-rights concerns, diminished but still present fears of a Vietnam- style escalation and other competing concerns are all obstacles."

Until the longest war in U.S. history, many policy makers did not realize that time is as much a strategic dimension in international conflicts as money and manpower for this democratic nation, especially in the age of instant international mass communications. In this respect, what happened in Vietnam can be summed up in a single prophetic statement of North Vietnamese Premier (later President) Pham Van Dong to journalist-scholar Bernard Fall in 1962: "Americans do not like long, inconclusive wars -- and this is going to be a long, inconclusive war. Thus we are sure to win in the end."

The loss of broad U.S. public support for the "limited war" in Vietnam, as measured by public-opinion polls, followed essentially the same basic course as the loss of support for the "limited war" in Korea, according to a study by Prof. John E. Mueller of the University of Rochester for the Woodrow Wilson Center's 1983 symposium on "Vietnam as History." Both wars were popular at the start but became increasingly unpopular over time as the battles went on and as U.S. casualties increased.

The intensity of public opposition to the Vietnam war, as Mueller noted, was much greater than had been the case in Korea. This was particularly true of the elite groups at the core of the foreign policy consensus. Among the most important and most lasting effects of Vietnam has been the shattering of the political consensus at home on foreign policy.

As a result of Vietnam, U.S. government officials -- and other nations, political groups and terrorist factions abroad -- are coming to understand that when American troops are committed overseas in "hot war" circumstances, a clock begins to run on political tolerance at home.

As codified by Congress in the War Powers Resolution resulting from Vietnam frustration, the president has 60 days, automatically extendable to 90 days, to take out the troops or obtain congressional approval to keep them in place. In fact, the situation is less clear-cut and more negotiable, but unlike the pre-Vietnam days everyone is aware that some limits are there.

The lightning U.S. military success in Grenada in October 1983 has been hailed by some as the end of a "post- Vietnam paralysis" of American military power. Some of the reasons for its acclaim at home were that Grenada was quick, inexpensive, daring and competely successful.

At the same time, however, the U.S. effort in Lebanon showed the classic Vietnam-Korea patterns to be very much alive. The U.S. effort in Lebanon was much smaller -- 1,600 Marines backed up by naval and air power with an initially modest peacekeeping mission -- compared to more than 500,000 in Vietnam and 300,000 in Korea. But as the military action in Beirut intensified, familiar pleas for "patience" and "determination" in "U.S. vital interests" were increasingly met by criticism that the mission was unclear, the means for accomplishing it inadequate and the operations flawed.

It took until September 1983, one year after the Marines arrived in Beirut, for Congress to invoke the War Powers Resolution. At that point, they authorized U.S. troops to remain for up to 18 months longer.

After 241 Marines were killed by a terrorist truck bomb in October 1983, pressure built in Congress and public opinion shifted against keeping the U.S. troops in place. Syria's President Hafez Assad, who remained intransigent in blocking U.S. goals, was quoted as remarking that the United States suffers from "shortness of breath," meaning that it has limited staying power. In February 1984, President Reagan suddenly cut his losses and pulled out the troops.

The heyday of American reluctance to intervene militarily abroad was from the Paris accords of January 1973 to the Soviet invasion of Afghanistan in December 1979, a little less than seven years. In the last year of President Carter and first four years of Reagan, the United States took a sterner view of faraway threats to its alliances and interests.

Neither this country nor the world has forgotten nor entirely outgrown what happened to the United States in Vietnam. As the years roll by, though, the blow to America's position recedes into the historical past.