TWENTYNINE PALMS, Calif. — As squads of Marines venture into the narrow, rutted streets of Wadi al Sahara, roadside bombs explode, women begin screaming in Arabic and snipers fire from windows.

Amid language problems and gunshots, the Marines must make instant decisions on returning fire and whether to storm a house — and, if so, with how much firepower.

But the Marines are not in Iraq, and the snipers are role players firing blanks. The Marines are in the desert of Southern California participating in "Mojave Viper," an intense 30-day training session in fighting an insurgency in which it can be difficult to distinguish militant from civilian.

"In a counterinsurgency, it's crucial that Marines have the tools to make ethical decisions on using force," said Lt. Col. Andy Kennedy, one of the officers in charge of the training.

Amid military investigations into the killings of Iraqis in the towns of Haditha and Hamandiya, questions about the Marines' adherence to the international laws of warfare and their own rules of engagement have gained new attention.

Although Marines here will not comment on the investigations, they say young Marines are being trained to realize that their actions in Iraq can have enormous consequences for the U.S. mission and probably will be scrutinized worldwide.

"We're taking 19-year-olds and teaching them to make split-second decisions about the use of force that are going to be analyzed on the 6 o'clock news," said Col. Ron Baczkowski, head of one of the training programs.

At their sprawling base, the Marines have constructed Wadi al Sahara, or Valley in the Desert, consisting of 475 structures, including a mosque and a "souk," or marketplace. Made from shipping containers, and spread over 360 acres, the faux village cost more than $23 million.

Many of the lessons of Mojave Viper are tailored to the evolving situation in Iraq, including the need for greater restraint in the use of deadly force compared with the assault on Baghdad, the Iraqi capital, in 2003 or the house-to-house fighting in Ramadi and Fallujah in 2004.

Role players, paid $150 to $275 a day, act the part of Iraqis, some friendly to the U.S., some hostile, some armed, some plaintive, some duplicitous. The woman sitting peacefully could be acting as a spotter for an insurgent sniper.

Role player Khalid Aledani, 40, a naturalized U.S. citizen born in Basra, says his goal is to teach the Marines to be alert but not afraid. "They need to know that not all the people there are bad," he said. "If you give disrespect to people, they will give you disrespect back."

Different scenarios test the Marines' ability to adapt. Stress is a good thing in training because in Iraq it can save the lives of Marines and civilians, Marine leaders said.

"I want to make sure we've stressed them enough," said Brig. Gen. Douglas Stone, commander of the Marine Air-Ground Task Force Command, which runs the village and the training course.

The scenarios are adjusted to conform to events in Iraq, including new ways insurgents are using roadside bombs or planning ambushes. When two U.S. soldiers were abducted and killed recently, the course was adjusted to include "lessons learned."

But the shift from traditional warfare to insurgency might be the biggest change that training at Wadi al Sahara has accommodated.

The training is meant to prepare Marines for one of the most dangerous and difficult decisions: when to storm a building where an insurgent is thought to be hiding. One of the investigations under way involves the killing of 24 Iraqis in Haditha, where Marines were searching, unsuccessfully, for an insurgent who had just detonated a bomb that killed one of their comrades.

In theory, the rules for "clearing a building" are simple: The person inside must have been conclusively identified as a combatant, and the threat must be confirmed as real.

But in practice, every case is different — particularly with an enemy that prefers to hide behind women and children, U.S. combat veterans said.

Marines who fought in Fallujah said later that the mere suspicion that a sniper was in a building was justification for calling in a tank or air strike. Now the bar is higher.

"It's not just one suspicion or one event (that is needed), but several," said Lt. Col. Pat Kline, deputy director of one of the training programs.

The presence of civilians also must be considered when deciding whether, for example, to enter a building by throwing in a grenade, as Marines did in Haditha.

"Because someone is hostile inside a house, that doesn't mean the entire house is hostile," Baczkowski said.

Every Marine battalion bound for Iraq goes through the Mojave Viper training, which ends with a 72-hour field exercise meant as a final exam to prepare Marines for the quick decisions that lie ahead.

Baczkowski said: "They have to be able to take in all considerations in that split second and be dead right — every time."

"We're taking 19-year-olds and teaching them to make split-second decisions about the use of force that are going to be analyzed on the 6 o'clock news."

Marine Col. Ron Baczkowski, on training troops to react in urban combat situations in which civilians are present.

Marines train to act quickly, but ethically