Sgt. Tyler Farmer

He first deployed to Iraq in 2004 and fought in Fallujah II. He talks about the tension, confusion and uncertainties faced every day in counterinsurgency urban combat. "If I need to pull that trigger to protect myself or fellow Marines, am I going to get in trouble for it?"

1st Lt. Alex Martin

At the Naval Academy he planned to become a pilot. After 9/11, he decided to become a Marine. To him, Haditha seemed a "beautiful village" at first, but his time there changed his perspective on everything: "The biggest takeaway from Haditha -- there's no bad or good, happy or sad. It's all of those things."

Capt. Adam Mathes

He was on his third Iraq deployment when he spoke to FRONTLINE. In 2005 he was in Haditha and before that, Fallujah. He talks about what Fallujah was like. "Combat doesn't recognize privilege, where you grew up. It's equal opportunity in its purest."

Sgt. Tim Tardif

He's been deployed to Iraq four times and talks about the impact of the Haditha investigations on himself and on other Marines. "We think about it all the time. Am I making the right decision? And am I doing the most that I can to keep my Marines safe?"

Hearts & Minds Training -- Inside Mojave Viper

In this video, a group of Marines practice "making a house call" on an Iraqi family. Paid actors, many of them Iraqi-Americans, role-play the family members, and the "home" is a converted shipping container, as are the other buildings in the two simulated towns set up by the Marines in the California desert.

Since September 2005, Marines shipping off to Iraq have practiced scenarios like this one at the Marine Air-Ground Combat Center at Twentynine Palms, Calif. (A second training facility in Bridgeport, Calif., more closely resembles the mountainous terrain of Afghanistan.)

The role-playing is part of a 30-day training exercise called Mojave Viper, which aims to simulate, as closely as possible, the conditions Marines will face when deployed to Iraq. Urban combat is only one-third of the program; Marines also train as a unit and learn how to call in artillery and air support as they would in combat. Learning to identify improvised explosive devices (IEDs) is also an integral part of the curriculum.

The paid actors are not the only Hollywood touch. Pyrotechnics and stage makeup are also used to simulate the aftermath of insurgent attacks. The verisimilitude culminates in a final, 72-hour simulation of operations in an Iraqi city, in which Marines must interact with local government and police while battling insurgents among the civilian population.

According to Marine Corps Commandant Gen. James Conway, the attention to realism has paid dividends in theater. "We've had units go through that have then gone to Iraq call back and say, 'You know, nothing I've seen here surprises me, based on our Mojave Viper experiences,'" Conway told FRONTLINE.