Near San Diego, California, soldiers, sailors and airmen are training for a possible war in Iraq. Some training involves realistic enactments of events that instructors hope will never happen.

In the dusty rolling valleys of this immense Marine base, Marine Corps trainees and Navy medics put on gas masks and protective gear.

"Go. Grab the gear, give it to your team. Go!"

Navy Lieutenant Nil Llagas says the masks, gloves, boots and outerwear the trainees are wearing are special. "This is what they call a chemical suit," he explains. "It protects them from chemical weaponry when they get into a chemical-infested area. They also learn how to decontaminate themselves so before they go into a safer zone, that they have been decontaminated."

With their gas masks tightly fitted, the trainees enter a building that instructors call the "gas chamber." Inside, a harmless gas called CS is released. Like tear gas, it burns the eyes and nostrils. If the masks are properly fitted, there is no problem, at least at first. Inside, says Marine Lance Corporal Jenna Vaughn, trainees do five minutes of calisthenics, then briefly take off the masks.

The sensation is unpleasant. "Burns and makes your nose feel runny. Makes you feel like you're sick. Gets rid of all the fluids in your body, basically," says Vaughn. "Just a little CS," the instructor responds. "You're going to get some more. Don't worry."

Gunnery Sergeant Robert Andrews is an instructor in nuclear, biological, and chemical warfare defense. He says it is easier to defend against chemical agents than against a nuclear blast, but adds it is possible to reduce the risk of radiation exposure, especially from dirty bombs, or bombs that use conventional explosives to disperse radioactive material.

"When it comes to radiological dispersion devices, like a dirty bomb, those are actually fairly simple to clean up," he explains. "You can remove the top layer of soil, you can decontaminate the buildings and the personnel."

Combat is never easy, says Marine Corps instructor Luther Catchings, who served in the Persian Gulf in 1991 during Operation Desert Storm. He fought against Saddam Hussein's elite Republican Guard, and says he does not underestimate his opponents in that conflict. He says he came to respect them as professional soldiers.

He says there is no substitute for combat experience, but in recruit training and later field exercises, instructors can help their troops sharpen their responses to situations they may encounter.

"They're doing this training, doing the same thing over and over. Sometimes you get tired of doing it, but what they're doing, what we're doing, is building muscle memory. So you don't have to think, you just react," he says.

Sixty-five kilometers south of Camp Pendleton, at Naval Station San Diego, training continues on board the guided-missile destroyer John Paul Jones. The ship contains a state-of-the-art AEGIS warfare system and can carry some 90 missiles, hitting targets at sea, on land and in the air. The ship also has a sophisticated radar to monitor inter-continental ballistic missiles in space.

Commanding officer David Steindl says even in harbor in San Diego, the crew practices missile launches using simulators, from its combat information center deep in the ship. "Just yesterday, we ran an exercise where we conducted numerous Tomahawk missile launches, simulated. The only difference between a real Tomahawk missile engagement and the one we did yesterday is that there is no missile that left the ship. But all the button-pushing, all the computer screens were exactly the same," he says.

As things now stand, the USS John Paul Jones will not be deployed to the Persian Gulf, even if war should erupt there. But Ensign Derek Pedro, the ship's 24-year-old communications officer, says ongoing training makes him confident that the crew is ready to go, if it is called on.

"I'm not concerned," he says. " We have a really good crew, a group of people on board this ship that know their job and they know the stuff they're working on. Everyone knows how to do what they've been taught to do, and we do it well."

Career armed services personnel say many have been in the military for years and never seen combat. But with repetitive training, says Marine instructor Luther Catchings, soldiers and sailors know how to respond when the bullets are flying.