Rona Fields slumped into the back seat of the jitney as it passed a military checkpoint on the road from Sidon to Beirut. Crammed together with a near-term pregnant woman, a 14-year-old girl and three other women, the American psychologist from Virginia held her breath as her Lebanese companions hid her.

In November 1982, Lebanon was on a short fuse. Two months earlier, U.S. Marines had landed in Beirut, where the streets exploded with relentless bombings and gunfire, and Americans become prime targets.

As the driver maneuvered past rubble entering Beirut from the south, several explosions rocked the vehicle. The driver slammed down the accelerator to outrun the attack. The women sobbed. The hysterical girl clung tight to Fields, who was surprised to find herself repeating gently in Arabic, "It's all right, it's all right, as she shielded the other passengers with her own body."It happened so fast, I really didn't have time to think about what I was doing," says Fields, who for more than two decades has traveled the globe to study children growing up in strife-torn societies. "Lots of times, I used to think about what I would do if ever and when something happened. But nobody ever knows until they face it."

With the outbreak of war in the Persian Gulf increasing the odds that Americans at home and abroad will face terrorist attacks, many people are wondering what they would do -- if ever and when. They find themselves replaying horrifying scenarios in their minds: What if a car bomb explodes near me? What if my family is caught in the cross-fire? What if I'm taken hostage? What if my plane is hijacked? The threat of terrorism suddenly has gone first-person singular.

Local mental-health professionals report that they already have seen increasing tension from the terrorist threat in their patients and in the public. "There's a lot of anxiety," says Stefan Pasternack, a professor of psychiatry at Georgetown University who has worked with victims of terrorism. "It is showing up in thoughts and feelings and dreams and concerns. We're seeing it and hearing about it."

Fields sees similar stress building up in her patients -- people she describes as "mostly average people" who live and work in the Washington area. "A lot of people are uneasy expressing all of the discomfort they have," she says. "Who wants to be taken for a paranoid? But this is something we have to think about."

Experts in terrorism and the psychology of violence say there are ways of thinking and acting that help to keep foreboding fear of terrorism in check and thereby undermine the purpose of the terrorist -- which is to terrorize. Foremost, they agree, is that Americans need to understand the threat for what it is.

"We do face the possibility of terrorist activity," says Brian Jenkins, author of the 1985 book "Terrorism and Personal Protection" (Butterworth, $55). "And if we think of terrorism as comprising not only the terrorist action, but also the atmosphere of fear and alarm resulting from the terrorist threat ... then we've had terrorism already in the United States for a couple of weeks now.

"But it is important to keep the threat in perspective. There is public speculation about targets, security is increased, there's an atmosphere of alarm. But ... we don't want to create Fort Apache without any Apaches."

Jenkins, who has researched "critical violence" for 18 years and is a senior managing director at Kroll Associates, a Los Angeles-based international investigative and consulting firm, says that in a typical three-week period in the United States, between 4,000 and 5,000 people die violent deaths -- from automobile accidents, homicides and suicides. "That is a predictable statistic," he says. "But because terrorism is different, because it is intended to create an atmosphere of alarm, we will focus our attention not on those predictable deaths, but on its unpredictable possibilities. And there's a danger in that," especially for Americans who are novices when it comes to such attacks at home. "We may be inclined," he says, "to overreact."

Psychiatrist Pasternack reminds his patients who exaggerate the threat that there is still much greater danger of them getting killed from a random drug shootout than from a terrorist action. The concrete illustration quickly balances their misgivings and puts terrorism into a more realistic perspective. "There's an old line from medical school," he says. "When you hear hoofbeats, don't think of zebras, think of horses. So, chances are that the loud noise you hear is a car backfiring. Sure, check it out. But keep cool. We have to because otherwise we make ourselves victims of our worst fantasies."

Another healthy response to the threat, which is designed to paralyze populations, is simply to act. That doesn't mean "doing outlandishly foolish things or sticking yourself in harm's way," as Pasternack warns against. It means participating in security efforts.

"Terrorists can attack anything, anywhere, any time, and we cannot protect everything, everywhere, all the time," says Jenkins. "That means ... the public has to participate. How? By understanding and going along with the modest increases in security that we are seeing. ... This is not a time to be rude to the poor guard who has been instructed to tighten things up" at the airport or office building.

To keep in mind that authorities who specialize in tracking terrorists are on alert also helps to defuse the terrifying fantasy that it's a them-and-me situation. "Obviously officials are taking a variety of steps to counter terrorism," says Douglas Simon, a Drew University political scientist, who 20 years ago, as an Air Force intelligence officer, dodged ambushes in the streets of Saigon. "It's disconcerting when it happens. There isn't too much people can do other than use common sense and roll with the punches."

Special Agent Jim Mull, a spokesman for the Washington Metropolitan Field Office of the FBI, suggests people be "a little more alert of their environment and surroundings. If you see anything strange or suspicious, don't think it is foolish to report it. ... If we find information that leads us to a terrorist act, you can be sure we're going to react. The FBI is only as good as the information it develops and receives."

That kind of vigilance is a lesson Americans can learn from Europeans who know terrorism firsthand. "Drop a shopping bag on a street corner or in a department store in Paris or London or Tel Aviv and somebody is going to notice it quickly and notify the authorities," Jenkins says. "This is about the level of awareness that we would have in terms of dealing with other dangers in our society: When you get on the expressway to drive {or} places we avoid going at certain times in order not to become victims of ordinary crime. We don't regard these as extraordinary measures. To a certain extent, the issue is to incorporate some of that same level of awareness into our understanding of terrorism without creating paranoia."

Not crossing that line from prudence to paranoia is critical. Irene Frieze says potential victims of violence sometimes avoid that trap by developing personal safety guidelines for themselves. Whether that includes altering one's daily commuter route to work to avoid likely terrorist targets or looking inside a bank before entering, "just having those rules seems to help people feel better," says Frieze, a social psychologist at the University of Pittsburgh. "You are better off having the sense that you've done what you can, and then just not worry about it."Other mental-health experts talk of digging deep within one's self for a personal sense of security. For Rona Fields, the jitney incident in Beirut became a "fixed point" of confidence, a calming acknowledgment of her own ability to survive a lethal situation. "I was proud of myself," Fields says. "I said to myself, 'Hmmm, so that's how you behave when you're in serious trouble!' I feel like now I instinctively know what to do and I will do it if I trust my instincts."

Anyone troubled by the terrorist threat would do well to recognize their own fixed point of personal strength, she advises. "If you have something like that in your life, an incident when you knew your instincts were good and you could trust them, use it. Every one of us has survived so far despite adversity. That's a source of strength -- the belief that we're going to survive again the next time."