The murder of three American medical missionaries in Yemen Monday is about the saddest thing one can imagine -- people martyred as they were helping others -- but it's not the hardest thing to imagine. After all, we are now in the era of asymmetrical warfare, where any American civilian anywhere is at risk.

This act was the essence of asymmetrical warfare. But wait a second, one might protest. Wasn't the man who killed those people, the man who disguised his rifle as a baby, a terrorist -- a mere criminal? How can one call him a warrior? Maybe it's because that's what the suspect calls himself -- a warrior for God, out to "cleanse" his religion. Americans can choose to define jihadists as terrorists, but not everyone else accepts our definitions. Indeed, many think that "terrorists" are not only warriors, but heroes. Today, "Osama" is one of the most popular names for newborn boys all across the Muslim world.

International polling data underscore the intensity of anti-Americanism, from which militants draw their strength. A Gallup Poll released in February 2002 surveyed Muslims worldwide and found that 53 percent viewed the United States unfavorably. Respondents described the United States as "ruthless, aggressive, conceited, arrogant, easily provoked, biased." A poll from The Pew Center, released just last month, found that the numbers have worsened. In Egypt, for example, a country that's been getting $2 billion a year in U.S. aid for a quarter-century, the U.S. favorability/unfavorability ratio was 6 to 69.

So is this the "clash of civilizations" that Harvard political scientist Samuel Huntington first warned about in 1993? Maybe. And, if it is, when will it end? Most Americans, including then-President George H.W. Bush, thought we were done with Iraq in 1991, and yet here we are a decade later, poised for another round. And, even after we "finish the job" in Baghdad, who can say how long we'll be skirmishing with guerrillas, the way we were in Vietnam -- or the way the Israelis are fighting today in the West Bank and Gaza? And, of course, if the Mideast erupts in the wake of an American attack, as virtually every Arab leader has warned that it will, we could discover that Operation Desert Storm II proved to be just the first phase of a long conflict ahead.

For those who hate America, or simply oppose its influence in their country, it's foolish for them to put on a uniform and declare, formally and overtly, war on the United States. Put simply, any such soldier will likely be struck dead immediately, via precision-guided munition, never having so much as fired a shot at an American. Recent history -- Sept. 11, 2001 -- shows that even those bent on their own kind of anti-American martyrdom are better off blending into the population, waiting for an opportunity to strike.

And the best strike opportunities for our foes are not likely to be wearing American military uniforms -- those are the "hard targets." By contrast, "soft targets" are just about everyone else. That is, you and me. Routinely now, the State Department warns American travelers to steer clear of hot zones in Africa and Asia.

Meanwhile, America itself gets hotter, or at least costlier. We are vexed this week by news of a manhunt for five mysterious enemies who seem to have infiltrated into the United States from Canada -- five needles in a haystack of 200 million cross-border travelers. But even if this or that terror alert comes to naught, the expenditures keep piling up. The Department of Homeland Security has a budget of $38 billion -- and rising. We are now spending billions preparing for new kinds of threats: anthrax, smallpox, surface-to-air missile attacks on jetliners, bombs aboard cargo tankers, sabotage against pipelines. And don't forget the new air-passenger baggage-screening system, just put in place. How much will that cost, in direct outlays and in indirect waste of flyers' time?

Americans don't mind bearing these burdens; they are being promised victory in the "war on terror." We have proved that we can win symmetrical wars. But this is an asymmetrical war. It's not a war against an army, but rather a war against a tactic, against resentment, against beliefs and ideas. And we have yet to prove we can win that.