Now-iconic photo by John Tlumacki of the Boston Globe, showing 78-year-old Bill Iffrig on the pavement Now-iconic photo by John Tlumacki of the Boston Globe, showing 78-year-old Bill Iffrig on the pavement

Americans spend a lot of time in crowds. We enjoy exercising our constitutional right to assemble freely.

Nine times so far this month, baseball fans have thrust themselves onto a Green Line train, like spam in a can, heading toward the Navy Yard stop so that they could join an even bigger crowd at Nationals Park. Washington this month celebrated the Cherry Blossom Festival, complete with a parade, and now today there’s another parade downtown, marking Emancipation Day.
Is there safety in numbers, or danger?

What do you do, day to day, in our increasingly urbanized world, when you know there are bad people out there for whom the killing and maiming of innocent people is not a tragedy but a successful outcome?

The answer, as most Americans know, is that you press onward. Life has to go on, terror or not. You can’t maneuver through life as if every garbage can holds an improvised explosive device.

America has been hit before, and the country will likely be hit again – and again after that. We’re the fat target, still the world’s lone military superpower, and the object of external and internal enemies whose bizarre thought processes have led them to believe that the only relief for their hatred is indiscriminate murder.

No amount of security and intelligence gathering is ever going to make us perfectly safe. This is an open society, not a police state, not yet at least, and so the best response to a tragedy like the one in Boston is to go on with your life, eyes open. Alert, but not paranoid. Like the signs say: If you see something, say something.

Until 9/11, the American people felt comfortably isolated from the rage and chaos on distant continents. Geography has always buffered us, framing our land in two vast oceans. Our biggest terror attack before 9/11 had been a home-grown event, in Oklahoma City.

Perhaps there’s been a tendency the last few years, as the shock of 9/11 has faded, to be lulled into complacency. We’ve learned of terror attacks foiled, like the one in Times Square in 2010, when a car bomb was spied by street vendors. We rely for our sense of safety on the notion that our intelligence agents are at work, infiltrating the terror networks, cultivating informants. We read about drone strikes in distant lands that eliminate the masterminds.

But this is asymmetrical conflict, and we have to defend ourselves everywhere while the terrorists can strike anywhere. It doesn’t take a genius to make a bomb and kill people.

At the moment the Boston terror attack remains mysterious. Who did it? Why? To what conceivable purpose? Are there more bombs out there, undetected? Was this a domestic or international terrorist?

It is perilous to try to anticipate a motive. In the past, foreign terrorists have tended to target perceived military or financial targets, or major transportation systems. We’ve seen a shoe bomber (foiled) and an underwear bomber (foiled) on jetliners. A terrorist struck at Fort Hood in Texas. London’s subway was hit, and Madrid’s commuter train. At the millennium a terrorist planned to hit Los Angeles International Airport. And there was 9/11, of course, which struck at the icon of international trade as well as the Pentagon.

What purpose, however twisted, would be served by killing 8-year-old Martin Richard? The little boy was there to cheer on his father, William, at the marathon’s finish line. Now the boy is gone, and his mother and one of his sisters are hospitalized with what news accounts refer to as “grievous” injuries – and Monday we all saw, in those Internet images, what grievous looks like.

The news reports this morning say that police have questioned a 20-year-old Saudi man who has been hospitalized; his apartment has reportedly been searched. But suspicion can mislead us all, whether we’re amateurs or professionals. Repeatedly in big stories like this, the early information has evaporated.

Yesterday the New York Post achieved huge Web traffic for a story saying that 12 people had been killed, but as of Tuesday morning the official death toll stood at three (with another 17 in critical condition, among 176 casualties total). The Boston police commissioner announced that an explosion had occurred at the JFK library; not true. There were reports of many other unexploded bombs; not true.

Until these questions are answered, the airwaves and Internet and newspapers will be filled with speculation about the perpetrator and the motive. We hear that perhaps it was a right-wing anti-government person, making a point on the day that taxes are due. For some reasons, this is the week in April when bad things have been known to happen. Oklahoma City. Columbine. Now Boston. There are those who will toss in the fact that Hitler’s birthday is April 20. But it may simply be random bad luck, a cluster effect seen in any small data set.

If the Saudi national being questioned turns out to be the perpetrator, and there’s a link to an overseas terror network, then what happened in Boston is a continuation of the conflict that gave us 9/11, scaled down this time, and instead of four jetliners loaded with fuel the weapons were small enough to fit into a couple of garbage cans.

But maybe it was just one guy. A lone wolf with a private agenda. The hardest thing to stop. This could be more like a Newtown event — someone trying to achieve notoriety through mayhem — than anything with a political subtext.

Strong countries are resilient. They adapt and move on. President Obama, who vowed Monday to find the person or people behind the Boston terror attack, caused a ripple of controversy a few years ago when he told Bob Woodward that the U.S. could survive another 9/11.

“We can absorb a terrorist attack. We’ll do everything we can to prevent it, but even a 9/11, even the biggest attack ever . . . we absorbed it and we are stronger.”

That’s the cold-eyed view of the situation. Terrorists do not pose an existential threat to American society. They cannot win this fight by any logical metric. At best, they can achieve a psychological goal, of getting into our heads, making us fearful. But over time we learn to control our fears and adjust our expectations, recognizing that we are not invulnerable.

First we have to get up off the pavement, like that 78-year-old Boston runner, Bill Iffrig, the one who crumpled when hit by the shock wave of the explosion just a few yards from the finish line. He’s the man in the now-iconic photograph of the three police officers springing into action with an older man – Iffrig – on the pavement, facing away from the camera. Iffrig was on TV Monday night, describing what happened, his tone remarkably matter-of-fact:

“Everybody else is out there having fun and you got one or two people trying to destroy the whole thing, it’s hard to figure out. Terrorists, whatever they are…. I don’t have much use for it.”

This is why terrorists won’t win: Too many American like Bill Iffrig, who, when they get knocked down, get right back up again.

And yes, he finished the race.