Chris Stegherr served in the Marines and is now a security guard. (Bonnie Jo Mount/WASHINGTON POST)

“One,” Chris Stegherr says as he breaks his staring contest with a young doe.

He’s alone out here along the Potomac River, the only light a pure white gleam of moonshine, occasionally augmented by the flashlight he keeps on his black security belt, next to his Glock 9mm, Mace and handcuffs. His mission, as he calls it, is to watch, to be here if something bad happens, to patrol these 600 acres of manicured land with the same hypervigilance he once displayed as a Marine stationed in Iraq.

Stegherr’s battle zone now is a Loudoun County golf course that hugs an especially quiet stretch of the Potomac River. He earns $16 an hour guarding Trump National Golf Club, where members pay $75,000 just to join.

Before 9/11, Americans did not worry so much about random acts of terrorism, but now they find comfort in knowing that more than a million security guards — double the number in the nation’s workforce a decade ago — patrol shopping malls and power plants and work through the night to protect public spaces.

Falken Industries, a Manassas company that hired Stegherr, saw opportunity in the country’s new anxiety. In just eight years, it ballooned from zero to 150 guards, blanketing government buildings, embassies and corporate sites in Washington and its suburbs with guards trained for every conceivable disaster. The company protects bowling alleys and jewelry stores with the same kind of attention it gives its top-secret government customers.

But what kind of terrorist event could happen on a golf course, where the only sounds at night are crickets or a distant firecracker? Since passenger planes were turned into weapons, obscure possibilities have abounded. Stegherr’s rounds include close attention to a fenced area near the Potomac. Inside the chain-link wire is a water-filtration system — a piece of “critical infrastructure,” as it is known among homeland security types — that someone might view as a target.

Stegherr presses down on a golf cart accelerator and hums forward, looping around a course so familiar that he often kills the headlights and moves stealthily around the ponds and greens. He stops, waits, stares.

To stay focused through his long, lonesome shift, he counts deer, the running tally a score card for each night’s patrol.

“Two,” Stegherr says, and a big buck darts off into the woods.

His calling began to crystallize in the 10th grade at Apalachee High in Winder, Ga., when an announcement on the PA system led everyone to watch the second plane hit the World Trade Center.

They watched the twin towers fall — and fall again and again on TV — for hours. They watched the Pentagon burn, watched as its wall was charred in black.

In those first days of fear and resolve, he knew: He would go protect his country. At 17, he persuaded his parents to help him sign up for the Marines. “I’ve always been for the country, want to keep it safe. I figured, the more eyes, the better,” he says.

In Iraq, he learned that something can happen at any minute and that he must be ready. It is the quandary every service member faces, hoping for calm yet bracing for danger. His training makes him eager to see some action, at least once in a while.

“It’s nice sometimes to actually have to do something,” he says, “because then you can put your knowledge to use.”

Falken has trained him to react to nearly every possibility. He went through firearm classes, shotgun training, an executive protection course, hands-on combat practice. On the golf course, he hasn’t had to use any of those skills — yet.

He knows the golf course as though it were his own back yard: public parkland at one edge, townhouses on another, the road up to the clubhouse. There’s an abandoned building down by the river where gang members and immigrants sometimes go to drink and party. He has learned to smooth over the dirt at the doorway so he can tell quickly whether strangers have come and gone.

At night, under a field of stars, with not a person in sight, Stegherr stays alert, helped by an occasional swig of Mountain Dew.

“Three,” he says. “And four.” He flicks on his flashlight and there they are, green eyes staring back.

At one bend, he sees a light, hears a rumble. A vehicle? On the course? At this hour?

He floors the accelerator. The cart hums along, edging closer to the possible trouble.

He pulls up alongside the cart and finds a prosperous-looking father, two young sons and a few fishing rods.

“We’ve been fishing this pond for 20 years and there’s never been an issue,” says the trespasser, a golf club member who immediately invokes the name of its general manager. “There’s a difference between young families and terrorists.”

Calmly, Stegherr says his orders are to ensure that no one is on the course after 9 p.m. “I’m just doing what I’m told until they tell me not to,” he says.

The member is hot and gunning for an argument. Stegherr tells him about the abandoned shed and how his job is to stay alert to intruders.

The member retorts, “There’s a difference between families who pay $75,000 a year to be here and guys out there drinking.”

After a few tense minutes, Stegherr bids the man good-night, and their golf carts part ways, threat averted.

“Five,” Stegherr says. It’s deer numbers five, six and seven, really — two fawns with their mother.

His life back in the States has a certain order to it. He sleeps a few hours in the morning after work; runs errands; maybe has dinner with his wife, Laura, a Navy lieutenant. Some days, he works afternoons at a jewelry store at Tysons Corner. He might grab a nap before heading to the golf club in the evening.

Once, he planned to stay in the Marines, but after he married into the military, coordinating careers became too complicated.

Now, at 26, he thinks about police work. He’s prepared, ready for action, hoping that he’ll eventually find the same sense of purpose that led him to the Marines in those frightening months.

But for now, the golf course gives him a small slice of America to watch over.

His cart slips behind a sand trap, then up past a water hazard. There’s a splash in the distance — a frog taking a midnight swim.

“Eight,” Stegherr says under his breath, as he looks into the darkness. The bushes rustle, and an animal scurries away.

First in the series: 9/11 widow still trying to find her new normal

Second in the series: Twin misses his other half

Third in the series: Brought together by catastrophe

Fifth in the series: Flight attendant still feels at home up in the sky