THE SHOWDOWN in the sand is not yet a month old and already there are muttered doubts inside the Beltway and beyond: How long will American public opinion support our massive military presence in the Persian Gulf -- and war, if it comes to that?

Six decades of polling suggest an answer to that question. This research also serves to dispel several myths that currently misinform much of the debate about our role in the Middle East. Here are six of these myths:

Myth 1 -- Every war is different.

Actually, our reaction to wars is suprisingly consistent: Public support appears to rise and fall in remarkably predictable ways. This is particularly true of limited wars fought in distant parts of the world.

The evidence comes from extensive analyses of data collected during the Vietnam and Korean wars. Studies of public attitudes toward those two seemingly different wars have produced a remarkable finding with decidedly unexpected implications.

Of all the complex variables governing public opinion, the single overwhelming factor is the casualty total. As the number of dead, wounded and missing in action rose, support for both wars fell at an identical rate. Moreover, this simple relationship explained virtually all of the change in attitudes during both wars -- suggesting that initial support for the war, the vagaries of domestic politics, different levels of public dissent and a host of other factors have little to do with shifting public attitudes.

"Casualties were easily the most important variable," said John Mueller, professor of political science at the University of Rochester and author of the groundbreaking book, "War, Presidents and Public Opinion." "We found that as casualties rose from 100 to 1,000, support for the war dropped 15 percentage points. As casualties rose from 1,000 to 10,000, support declined by another 15 percentage points." Should casualties increase to 100,000, the model suggests, support would drop another 15 points.

Of course, ideological predisposition also affects the national mood; and in that regard, support for the Gulf crisis could erode faster. Korea and Vietnam were wars that initially sounded "right" to Cold War America. Whether protecting access to Mideast oil would play as well as the Red card did in Korea and Vietnam is an open question: "Going to your death for stable oil prices might be difficult for the public to accept," Mueller argues. " 'He died for $25-a-barrel-oil' -- you don't put that on somebody's tombstone."

Myth 2 -- Stalemates quickly become stale.

If the shooting hasn't started -- and it hasn't yet -- there is little historic evidence to suggest that support for Bush's desert gambit will weaken substantially in the near future.

The United States has kept troops in South Korea for four decades with hardly a whimper from the body politic. In 1979, as the U.S. military prepared to celebrate its 30th anniversary in South Korea, just a third of those interviewed in a Gallup poll favored withdrawing troops.

Moreover, this country currently maintains 320,000 soldiers in Western Europe and the public seems in no partiuclar hurry to bring them home. The U.S. contribution to NATO, for example, is running about $160 billion annually (a somewhat misleading figure since some of these forces would be based elsewhere even if we left Europe). By contrast, the estimated annual cost of Operation Desert Shield is $10 billion to $15 billion.

Of course, there are circumstances under which support could shift dramatically. Operational blunders could turn Desert Shield into a Johnny Carson punch line. Iraq could commit atrocities that would further steel public support and perhaps lead to calls for massive retaliation. Or America's currently soft economy could plunge into deep recession -- perhaps the single greatest threat to public support for a costly Mideast buildup.

But until the killing starts or the economy dies, it's a good bet that reporters and soldiers sweating it out in Saudi Arabia will tire of a stalemate long before the public.

Myth 3 -- In times of war, we aren't Republicans or Democrats, we're Americans.

The public does rally around the president in times of crisis. But when the going starts getting tough, the opposition party gets going.

Polls conducted during the Korean and Vietnam eras suggest that party membership dramatically affects individual perceptions of the overall success and failure of the war effort. Gallup data from the Truman and Johnson administrations shows that Democrats were less likely than Republicans to be pessimistic about the progress being made in each war.

Trend data collected during Vietnam discloses that support for the war among Democrats and Republicans abruptly shifted when LBJ moved out of the White House and Nixon moved in. Before 1969, a larger percentage of Republicans than Democrats opposed the war. Thereafter, however, Republicans were more inclined to support the war than were Democrats.

"It's the 'my team' effect, and you see it again and again in a number of areas, but frequently in foreign policy," said political scientist Everett Ladd, who directs the Roper Center for Public Opinion Research at the University of Connecticut. "There is a section of the public, and not a trivial section, more inclined to grant consent when it is the president of their party who is carrying out the activity."

Ladd suggests that partisan differences may therefore be among the first cracks to appear in Bush's solid shield of public support, as strong Democrats begin responding to the first discouraging words from their party leaders. Those words may begin to be heard next week when Congress returns from its summer vacation.

Myth 4 -- Americans need a strong reason to believe in a war, and they don't yet have one in the Middle East.

(Actually, this is a near myth.) It would be nice to think this is so, but history indicates otherwise. Public opinion of recent wars -- even those the populace doesn't completely understand and will later reject -- typically starts high.

The currently popularity of the reason-to-believe myth can be traced to Vietnam. In 1967, Gallup asked Americans this question: "Do you feel you have a clear idea of what the Vietnam war is all about -- that is, what we are fighting for?" Fewer than half -- 48 percent -- thought they knew.

To many, this result suggested that the government had failed to justify the war to its citizens. That could be true. But there may be another explanation: Americans typically are puzzled by or simply ill-informed about foreign relations, including war policy. Thus the 48-percent figure may reflect acknowledged ignorance.

Consider what happened when Gallup asked the same question six months after the Japanese attacked Pearl Harbor. Barely half -- 53 percent -- of those interviewed said they had a clear idea of why the United States was in World War II. (The percentage did rise considerably later in the war.)

Anyway, worries about a befuddled public may be moot: According to a new Washington Post poll completed last week, 70 percent of those persons questioned said they had a clear understanding of why Bush sent troops to Saudi Arabia.

Myth 5 -- You can't fight a real war in the television age. Some researchers argue that this is one of the longest-lived myths of the Vietnam war -- one that should have expired 15 years ago when studies began showing that increases in casualties had almost precisely same the effect on public attitudes toward Vietnam -- America's first TV war -- as they did for Korea, perhaps our last newspaper war. (This apparent mathematical fact of life has been confirmed and amplified by numerous studies.) If television had a pronounced effect on attitudes, the rate of decline during the Vietnam War should have been greater, Mueller and other social scientists say.

"War, after all, is a singularly unsubtle phenomenon, and the assumption that people will know how they feel about it only if they see it regularly pictured on their television screens is essentially naive and patronizing," Mueller has written.

There even is some admittedly shaky evidence that watching television news accounts of the fighting and dying in Vietnam may have momentarily increased support for the war. A 1969 Harris Poll asked: "Has the television coverage of the war made you feel more like you ought to back up the boys fighting in Vietnam or not?" This somewhat loaded question produced a hawkish 83 percent affirmative response. Even those unwilling to dismiss the impact of TV acknowledge that evidence for its effect is, as Ladd says, "very, very hard to find."

And even assuming that televised protest did play a signficant role in determining public support, it is unlikely that the always-photogenic peace demonstrations of the Vietnam era would occur in these days of an all-volunteer army.

Myth 6 -- Americans don't like being the aggressors and won't tolerate an offensive war against Iraq.

That's what the public seemed to be clearly saying to pollsters immediately after Bush sent troops to the Gulf. But it probably isn't what they meant. In fact, Bush probably has more publicly acceptable policy options than the first polls may suggest.

Except in extraordinary circumstances, most Americans dutifully follow the commander-in-chief on matters such as foreign relations where the public has little knowledge or firmly held views. Remarkably, this tendency persists even when Americans initially may have doubts about the direction in which they are being led.

A classic example occurred just last year. An October Gallup survey for Newsweek asked respondents whether they favored or opposed using "U.S. military forces to invade Panama and overthrow Noreiga." Only 26 percent supported committing troops.

Barely two months later, Bush did precisely that. And the reaction was immediate: 80 percent of those questioned in a Gallup Poll days after the invasion supported Bush's actions; just 13 percent were opposed.

Panama was, at best, a war writ exceedingly small. Vietnam, however, provided equally graphic examples of how the public in the early years of the war, when casualty levels were still tolerable, readily accepted official war policy.

In October 1967, a Gallup poll found that about two out of three Americans opposed de-escalation of the war in Vietnam and a slightly smaller majority rejected withdrawing troops. But in another poll taken three months later -- when those options were presented as an official change in policy -- opinion followed policy.

Gallup asked: "If our government were to decide at this time that the best thing for us and the Vietnamese to do would be for the United States to stop the bombing and the fighting and gradually withdraw from Vietnam," nearly six out of 10 said they would approve of this new policy and barely a third said they would oppose such a plan.

Likewise, when Harris asked in September 1965 if the administration was right or wrong not to bomb Hanoi or Haiphong, just 30 percent approved. A scant 10 months later -- but after the bombing began -- 85 percent said the administration was right.

Similar effects may have been at work in the Civil War and World War I, said Mark Lorell, a senior defense analyst at the Rand think tank and co-author of "Casualties, Public Opinion and Presidential Policy during the Vietnam War," a 1985 study for the Air Force. Lorell said each conflict was preceded by widespread public protests in the countries involved. Yet the eventual declaration of war was immmediately met with equally ubiquitous support. What does all this suggest for the current conflict? It would seem to imply that Americans now support a defensive crouch in the Persian Gulf and oppose an invasion of Iraq because that's what their president has done. Should Bush decide tomorrow to invade Iraq and offer a plausible explanation to justify this decision, he could expect support "until the casualties start coming back," Lorell said.

Set against the public's historic predisposition to follow and support official policy -- wherever it leads -- it may be grimly comforting to note the inevitable braking effect that body counts seem to have on public opinion and, eventually, on public policy. It may also be discomforting to note that this lesson was known but ignored in the first years of the Vietnam war.

The relationship between casualties and public support for the war was specifically discussed before the National Security Council on July 21, 1965. Undersecretary of State George Ball, quoted in Rand's study for the Air Force, recalled what he said at that meeting:

"In a long war, I said the president would lose the support of the country. I showed him a chart I had prepared showing the correlation between Korean casulaties and public opinion, as our casualties during the Korean war had increased from 11,000 to 40,000. The percentage of those Americans who thought that we had been right to intervene had diminished from 65 percent in 1950 to a little more than 30 percent in 1952. Moreover, as our losses mounted, many frustrated Americans would demand that we strike at the very jugular of North Vietnam with all the dangers that entailed."

Ball recalls telling President Johnson directly: "Look, you have a lot of support right now, but once you get a lot of casualties, this thing is going to change." But, Ball recalled, "nobody really focused on the consequences of a lot of casualties." Richard Morin is The Washington Post's director of polling.