“The man who jumped over the White House fence and sprinted through the main floor of the mansion could have gotten even farther had it not been for an off-duty Secret Service agent who was coincidentally in the house and leaving for the night. … He happened to be walking through the house when chaos broke out and the intruder dashed through the main foyer …”

—The Washington Post, Sept. 30, 2014.

First, we thank the off-duty Secret Service agent for doing on Sept. 19 what several on-duty agents had not: He tackled an intruder, armed with a knife, who walked in the front door of the White House, flouting five rings of security.

So much for all those reassurances through the years about rooftop agents armed with sniper rifles, missiles and fail-safe judgment. On Tuesday, we also learned, from the (now former) head of the service herself, that before Omar Gonzalez displayed his track-and-field skills, at least two of her uniformed officers recognized him from Aug. 25, when he had been stopped near a fence south of the White House while carrying … a small hatchet.

What’s most remarkable is that by one metric this systemic blunder is only the most egregious in a long list: Gonzalez apparently is at least the 29th person in the last 40 years to have breached the White House perimeter — alone. And we’re not counting the uninvited social climbers who notoriously waltzed through two security checkpoints at a 2009 state dinner for India’s prime minister and, after waiting their turn, shook hands with President Barack Obama. Nor are we counting the toddler who, on Aug. 7, squeezed through a White House gate and was returned to his parents.

Throughout our history, our headlines and our popular culture most often have warned us to beware of ill-inspired groups, from the outlaw gangs of the Old West and the Ku Klux Klan to today’s al-Qaida, Russian gangsters and the Islamic State. Time and again, though, one-off acts such as that of Gonzalez show us the astonishing power of the individual to wreak varying degrees of mayhem. A lone national security worker, having pirated supposedly sacrosanct computer data, flees the U.S. to expose spy programs. A lone man allegedly bent on suicide causes enough damage to disrupt days of air travel. And in scenes repeated nightly in Chicago, a lone gunman tugs a trigger, hoping he’s skilled enough to snuff out another life.

We don’t equate the culpability of these and other individuals who break ranks and cause havoc: Some are nitwits, some are brilliant, most have led lives that are variations on average. Their motives are grounded in excesses that range from anger to derangement, from revenge to sabotage, from visceral grudges to misguided mischief. What typically empowers them is their very aloneness: A squad of terrorist intruders has less chance of invading the White House than does one man guided by a goofy muse that the front door might be unlocked.

If you’re waiting for the passage where we opine that we cannot stop every squirrelly plot and thus always risk being victims, this is it. With this codicil:

Even as a superpower, the U.S. cannot stop every global evil. But we have chosen — over years, decades, centuries — to stand up to some brutality, the better to discourage other brutality. Friends and foes worldwide have spent 238 years discerning that America generally is vigilant, diligent, resolved. Yet the shocks of Sept. 11, 2001, managed to smack one generation the way Pearl Harbor smacked another in 1941, and the way P.G.T. Beauregard’s bombardment of Fort Sumter smacked another in 1861.

As individuals, too, we cannot stop every evil. But 9/11 reawakened an American sensibility not witnessed broadly since World War II: If each of us is vigilant, diligent, resolved in our daily movements and encounters, we just might be able to stand up to something nefarious, or discourage something nefarious. In New York City and elsewhere, the civic imperative has been reduced to six crisp words: If you see something, say something.

The intent is obvious — to help each of us subordinate the impulse to think that something bizarre or threatening or merely out of place probably isn’t a problem waiting to happen. Too often, though, it is. Poking fun at the Secret Service is America’s new parlor game. But what’s most alarming in the Sept. 19 incident isn’t that Gonzalez made it into the White House, but rather Tuesday’s disclosure that two guards recognized him before he entered the grounds. Given that, how did Gonzalez get as far as he did? We hope to learn more about what did, and what didn’t, happen in the succeeding seconds and minutes.

This said, the White House incident also demonstrates why vigilance, diligence and resolve are often rewarded. The Washington Post’s reporting that Gonzalez had been able to sprint past a stairway that leads up half a flight to the first family’s living quarters is alarming. What’s heartening is that one off-duty agent, who had just seen Obama family members depart by helicopter, was vigilant enough, diligent enough and resolute enough to take down Gonzalez.

May each of us, in our own small ways, take our own intelligent measures to keep one another safe.

Editorial by the Chicago Tribune