BAGHDAD - One mistake and the suicide bombers and assassins win.

That simple reality drove the soldiers of a Fort Carson battalion guarding the Green Zone along the Tigris River here, motivating them through grinding hours of mind-numbing repetition Sunday as they searched every car and every person coming through the gates.

The 600 soldiers with the 3rd Battalion, 29th Field Artillery Battalion have drawn one of the least glamorous yet most important jobs in the war zone. They alone protect the Iraqi government and the U.S. Embassy from the violence of Baghdad.

On a typical day, they process nearly 10,000 pedestrians into the government compound and search another 7,000 cars.

“It’s a drill they do every day,” Capt. Stephen Walker of Jersey Shore, Pa., said as government workers formed a block-long line Sunday to begin the Iraqi workweek. “It’s a true test of discipline.”

The Green Zone, more formally called the International Zone, remains the safest place in war-torn Iraq, allowing the government to move ahead with a minimum of insurgent fears. Because of its strategic and symbolic importance, the complex is a constant target for suicide bombers and assassins who want to vent their rage on the country’s leadership or top U.S. officials.

That puts pressure on the American-run checkpoints, where Fort Carson soldiers must stay perfectly polite to the Iraqi leaders and bureaucrats, while performing meticulous checks on everyone entering.

Soldiers greeted the workers Sunday with friendly waves and even a few pleasantries in Arabic as they guided them through the process, which takes as long as 20 minutes for each person entering the government zone.

“You’ve got your 99.99 percent of these who are hardworking people,” said Staff Sgt. Jason Obermuller of Tampa, Fla. “They aren’t all criminals.”

But that doesn’t mean they get a free pass. Everyone is checked several times, by Iraqi soldiers, American contractors and finally the Fort Carson soldiers.

Every purse is perused. Metal detectors probe pockets. Trunks and glove boxes are scanned by wary American eyes and sniffed by bomb-detecting dogs.

“You’re going to follow our rules, but we will treat you with respect,” explained Capt. Frank Harmon of Cherryville, N.C.

The battalion is part of the 3rd Brigade Combat Team, which left Colorado for war in December. Its soldiers initially thought they would patrol Baghdad neighborhoods. Instead, they were assigned gate duty and have had to learn every aspect of security, from memorizing more than a dozen types of identification used in the Green Zone to learning how to use sophisticated gear to X-ray truck cargo.

Mornings are the busiest time for the soldiers, who open the gates at 7 a.m. and are greeted by early-bird workers already in line. Some of the workers Sunday brought doughnuts for the office, which were thoroughly examined but not eaten by the artillery troops.

“The first few hours, the line is huge,” said 1st Lt. Jason Chlebek as he watched the Green Zone workers, most of them wearing suits and ties, shuffle through the security line.

The Iraqis know the drill and have their identification cards handy. Some grumble at the searches but most are friendly.

They all know the damage that a suicide bomber can inflict in Baghdad, and they don’t want one visiting them at work.

Soldiers eye everyone in the crowd.

“You watch for signs of anything that could be trouble,” said Sgt. Henry Correa of San Jose, Calif.

The painstaking work quickly becomes a grind. Sergeants rotate soldiers through the varied jobs of the checkpoint frequently to keep everyone alert.

Soldiers say they have settled into the job.

“I’m so used to it by now that I’m just used to it,” said Spc. LaTasha Hunter of Selma, Ala.

Commanders say they worry about burnout in the ranks because the guard work can become tedious quickly. They keep the gate shifts brief for the soldiers so they can relax and stay sharp.

“We have good shift hours so we’re not pushing them all the way to the limit,” said Staff Sgt. Walter Mack of St. Thomas, U.S. Virgin Islands.

The shortened shifts mean troops work only eight hours per day, seven days per week.

The drudgery will really set in a few months as the mercury rises, commanders say. Temperatures topping 120 degrees are common during Baghdad summers.

But there’s no room for complacency.

“What we tell our soldiers is that they have to be right 100 percent of the time,” said Maj. Rick Robinson, the battalion’s executive officer. “If they (insurgents) make it inside the International Zone and they detonate one bomb, they have won.”