Belgian police stand in the distance as a bomb disposal expert in heavy protection gear arrives at the scene of an apparent operation against terror suspects near the Meizer roundabout in Brussels’s Schaerbeek district on March 25. (Julien Warnand/EPA)
Juliette Kayyem, a former assistant secretary at the Department of Homeland Security, is the author of “Security Mom: An Unclassified Guide to Protecting Our Homeland and Your Home.”

Admit it. After the terrorist attacks in Brussels this past week, after the brief reflection for those lost or wounded and the sense of “oh, no, not again” passed, other thoughts quickly followed. My own selfish but natural worry, as a mother of three: Should we cancel that trip to Europe this summer?

In the nearly 15 years since 9/11, the questions I’ve fielded from family and friends have varied but never ceased: Should I buy a gun? (Only with training and safety measures at home, and certainly not to combat Islamic terrorists.) Is Times Square safe on New Year’s Eve? (Like every crowd scene, you have to stay alert, but security is high at events like that.) Or my personal favorite, because it combines parental insecurities with disaster management: Is Tulane a good school so many years after Hurricane Katrina? (Yes; it had a few rough months, but your kid should still apply.)

All these queries about a world in mayhem boil down to: Is my family safe? The answer is both simple and liberating: No, not entirely. America was built vulnerable, and thank goodness for that.

The flow of people and things, the movement to and within cities, the congregation of the masses that makes our lives meaningful, whether at church or at Fenway Park, are inherently risky. Our system (a federal government with limited powers, mayors overseeing police departments, governors directing National Guards) wasn’t designed to produce a seamless shield against every conceivable threat. Every day, more than 2 million passengers board planes at U.S. airports. The movement of goods and services — the expectation that everything from airline tickets to groceries can be purchased with just a few mouse clicks — is our lifeline. We’ve traded a measure of safety for convenience. And in our America, there are sometimes monsters under the bed.

In the immediate years after 9/11, the security establishment sold a vision of an America that never existed, a vision based on some notion that the country had been invulnerable and risk-free before the terrorists struck. The shock of such a massive attack on civilians in the homeland caused much of the public to experience collective amnesia, as if our nation had never before navigated perils, including ones more dangerous than al-Qaeda. President George W. Bush sold this notion to Congress and the American people when he said, in a speech just a few days after 9/11, that the attacks were something “never before” seen and something, he implied, that our national response could prevent from ever happening again — that if only we fought back harder and bought more gizmos, America could reduce its vulnerabilities to zero. Not so.

Threats constantly change, yet our political discourse suggests that our vulnerabilities are simply for lack of resources, commitment or competence. Sometimes, that is true. But mostly we are vulnerable because we choose to be; because we’ve accepted, at least implicitly, that some risk is tolerable. A state that could stop every suicide bomber wouldn’t be a free or, let’s face it, fun one.

We will simply never get to maximum defensive posture. Regardless of political affiliation, Americans wouldn’t tolerate the delay or intrusion of an urban mass-transit system that required bag checks and pat-downs. After the 2013 Boston Marathon bombing, many wondered how to make the race safe the next year. A heavier police presence helps, but the only truly safe way to host a marathon is to not have one at all. The risks we tolerate, then, are not necessarily bad bargains simply because an enemy can exploit them.

No matter what promises are made on the campaign trail, terrorism will never be vanquished. There is no ideology, no surveillance, no wall that will definitely stop some 24-year-old from becoming radicalized on the Web, gaining access to guns and shooting a soft target. When we don’t admit this to ourselves, we often swing between the extremes of putting our heads in the sand or losing them entirely.

This is not to say we should throw up our hands and hope for the best; our sense of unease is real. Yes, gun-related deaths (firearms are responsible for more than 30,000 American fatalities per year) far outstrip any violence in our country associated with terrorism , and our chance of dying in a terrorist attack is 1 in 20 million — which is why President Obama says the Islamic State is not an existential threat. But it doesn’t help much to criticize the nervous and anxious simply because terrorism makes them feel terrorized. If that 1 in 20 million is my child, it’s an existential threat to me.

Yet we still live, often joyfully, in a world with gun violence. And drunk drivers. And disease. We implore government to allocate resources as best it can to minimize those risks. Once we move past our angst, this becomes the most rational way to approach terrorist violence.

Accepting these vulnerabilities means our safety can be measured and evaluated on three core premises: how well we minimize our risks, maximize our defenses and maintain our spirit.

Minimizing risk includes actions as far-ranging as intelligence-gathering, the disruption of terrorist cells abroad (by drone strikes or police sweeps), diplomatic efforts and criminal cases. It’s reasonable to debate the relative weight we should put on each, but risk will remain as long as our enemies do.

Maximizing our defenses includes implementing visa and border controls, supporting first-responder training and equipment, and hardening soft targets to the extent practicable. A bunch of state police officers watching a crowded bridge can often look like security theater, but its intent is to display a layer of defense that could make an attack less likely.

And, finally — essentially — we must maintain our spirit. Those who challenge xenophobia and Islamophobia are often derided as “politically correct.” If that means pro-security, then it’s a critique worth embracing. In the course of America’s history, the single attribute beyond geography that has made us safer is our capacity to absorb, integrate and assimilate communities that come to our nation and call it home. It’s an experiment we have not always lived up to, but it’s one that puts us on surer footing than our European allies. And to stray from that tradition would lead us in directions that are more dangerous than the one we find ourselves heading in today.

If the attributes of our homeland security sound familiar (risk-reducing, defense-building, spirit-maintaining), it is because we practice them every day at home. We lock doors, wear helmets and keep a fire extinguisher in the kitchen — but we don’t shy from cooking on the stove. Stuff also happens in the homeland. We must demand much from our government to make us safer. But we must also accept that vulnerability isn’t always failure.

Twitter: @juliettekayyem

Read more from Outlook and follow our updates on Facebook and Twitter.