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

Nine times so far this month, Washington 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 probably 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 such as 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 in the past few years, as the shock of 9/11 has faded, to be lulled into complacency. We’ve learned of terror attacks foiled, such as 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.

When it happened, the Boston terror attack was a mystery. Who did it? Why? To what conceivable purpose? Are there more bombs out there, undetected? Was this a domestic or international terrorist?

It is always perilous to try to anticipate a motive before charges have been filed. 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.

So why the Boston Marathon?

What purpose, however twisted, would be served by killing 8-year-old Martin Richard? The little boy was there with his family at the marathon’s finish line. Now the boy is gone, and his mother and his six-year-old sister suffered “grievous” injuries – and Monday we all saw, in those Internet images, what grievous looks like.

News reports initially said that police questioned a 20-year-old Saudi man who had been hospitalized; his apartment reportedly had been searched. But early reports calling that man a “suspect” appear to be incorrect, with more recent reports suggesting that he was merely a witness, and fully cooperating. Suspicion can mislead us all, whether we’re amateurs or professionals. Repeatedly in big stories like this, the early information has evaporated.

Monday 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 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 all of the questions are answered, the airwaves and Internet and newspapers will be filled with speculation, including theories about the date. For some reasons, this is the week in April when bad things have been known to happen. Oklahoma City. Columbine. Now Boston. But it may simply be random bad luck, a cluster effect seen in any small data set.

If it turns out there is 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.

What if 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.

Whatever the reason, what remains is that strong countries are resilient. They adapt and move on.

“The American people refuse to be terrorized,” President Obama said Tuesday morning, “because what the world saw yesterday in the aftermath of the explosions were stories of heroism and kindness and generosity and love.”

Obama caused a ripple of controversy a few years ago when he told Bob Woodward that the United States 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, foreign or domestic, 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 Americans like Bill Iffrig, who, when they get knocked down, get right back up again.

And yes, he finished the race.