CAMP LEJEUNE, North Carolina—It was over 100 degrees on a recent summer morning when a squad of young Marines in combat gear, soaked in sweat, scoured “the bad side of town” for a bag of cellphones bearing the enemy’s battle plans.

They spread out to secure the dirt roads and buildings of an Eastern European rural village that reeked of animal dung and gunpowder. Russian voices rang out from a squabbling throng of civilians in a narrow corridor: “Help us! Doctor!”

The Marines wanted to help, but their first priority was to protect themselves and then to secure their mission objective: the cellphones. Civilians’ needs came third.

One Marine stepped onto a broken patch of dirt concealing an improvised explosive device.

The boom echoed off the warehouse walls and thrummed into the Marines’ bones. The Marine who stepped on the IED fell to the ground and went limp.

“Dead!” someone shouted.

Marines dragged their fallen brother into an open-air medical clinic and took up defensive positions, yelling questions and orders across the courtyard. Then came loud pops from above: the sound of an AK-47 being fired. Some of the young Marines ducked behind a fake stone wall as others turned toward the sound and returned fire.

A reporter raised a cellphone to film the Marines. “Put your phones away!” a squad leader shouted, and the reporter did.

The crisis seemed to be winding down when a man in a sweatsuit sauntered across the courtyard, unconfronted by Marines doing triage. As he passed the open doorway of the clinic, he casually tossed something inside.

A grenade. The Marines in the clinic dove for cover as their squad leader tried to sweep it away, but the grenade hit a wall and stayed in the room.

“Dead! Dead!” a man in civilian clothes shouted, pointing at two of the Marines closest to the grenade. “Concussed! Concussed!” he shouted at two more. The targeted Marines slumped to the floor as their brothers fired into the courtyard.

The civilian was Marine veteran Greg Jackson and the Ukrainian village was the Immersion Trainer that Jackson runs.

“This is why we train,” he said of the grenade scene. “The four Marines in the front of the clinic would all be dead, and likely those in the back room would be down, from the compression. But it doesn’t help them learn if we just say they’re all dead.”

Marines in the base’s live-action combat simulator now regularly do battle with Russian-speaking insurgents in an unnamed Eastern European country, readying for the conflict one trainer said would be “World War III.”

What the contractor calls the “Eastern Bloc” scenario no longer seems as far-fetched, given Russia’s annexation of Crimea and incursions into eastern Ukraine, where non-uniformed Russian troops known as “little green men” are waging low-grade war against Ukrainian government forces.

A small number of American troops are already on the ground in Ukraine as trainers; they have established a combat training center in Ukraine, staffed by Ukrainians and capable of hosting an entire brigade.

In July, Congress approved a proposal drafted by the Defense and State Departments that would allow the sale of lethal defensive weapons to Ukrainian forces such as the Javelin anti-tank weapon system.

The grenade and IED in the simulator may have been fake, but the emotions from the Marines were not. In debrief sessions with trainers and squad leaders afterward, some were angry or near tears because they had made mistakes and let their brothers down. Every Marine was aware what such a mistake could mean in real combat.

“We lost four Marines today,” Cpl. Nathan Watkins, a squad leader from Logan City, Kentucky, said after a first run-through with a new squad of less-experienced Marines. “Someone didn’t watch out, and we collapsed somewhere… We could have done better. We’ve got a lot of work to do. I wish we could do this once a month.”

More opportunities will arise in 2020, when Camp Lejeune plans to open an outdoor facility capable of training three infantry squads, about 40 Marines, simultaneously. But for now, the indoor sessions are in high demand, maxing out at three platoons a day. Infantry units about to deploy overseas skip to the head of the line.

“I know in my heart that what we do here saves lives,” Jackson said. “There’s nothing we can’t simulate. And I’m impressed with [the Marines we train] every day. They surprise you, and they prove they have what it takes.”

Those combat surprises, good and bad, are recorded for training purposes. The facility uses 167 high-definition cameras to videotape every inch and angle of the action, producing a DVD that squad and platoon leaders can use as “game film.”

The smells of gunpowder and feces in the warehouse are artificial, pumped in by a “scent-generator” to simulate the sensations of a deployment in a war-torn village near the Russian border. Artificial sounds, from crickets to the muted booms and pop-pop-pops of far-off combat, are also piped in.

“They’ve got to be ready for anything,” Jackson said of the infantry and intelligence cells who go through the Immersion Trainer’s Eastern Europe scenario. “But truth is, if U.S. Marine infantry platoons are on the ground in Ukraine, then we’re in the middle of World War III.”

Role players are asked to seek personal information from Marines during training scenarios as a way to teach information security. Jackson recalled a “prostitute” in the Immersion Trainer who asked for and received a young Marine’s cellphone number during a session. Jackson entered the number into databases to find personal information about the Marine and other Marines connected to him on social media. After the session, Jackson revealed to the squad that due to that “honey trap,” he had most of their home addresses.

The Russian spoken by “local civilians” in the Immersion Trainer is close to what one would hear in an Eastern bloc country, said Chip Olmstead, deputy director for Range and Training Area Management. Key to the facility’s success in the five years since it opened is the acting ability of the people involved, especially those tasked with playing the roles of local populace.

“Without the role players, this is just a fancy facility with smells,” Olmstead said. “The people make it work.”

For years, the paid actors known as “role players” in the Infantry Immersion Trainer were American citizens who hailed from the Middle East, and who spent their days hollering and pleading at the troops in Arabic or Pashto, but rarely in Russian.

Like a movie studio production team changing scripts, sets, costumes, and actors, the simulator can be restructured to simulate other areas such as the Philippines or East Africa, where terrorist groups like al-Shabab and Jahba East Africa, an ISIS affiliate, operate.

During the peak years of the Iraq and Afghanistan wars, the simulator was used to train deploying troops on cultural sensitivity issues, how to conduct key leader engagements with local government, and the chaos that ensues once the first round leaves an AK-47.

In the newly re-imagined Eastern bloc town where local civilians speak Russian, these role players are mostly of Eastern European or Russian descent. They are first- or second-generation immigrants who live in the South and speak fluent Russian and other Eastern European languages. They are heavily vetted and, as Olmstead puts it, “very expensive.”

Not every role player acts as a civilian. Some play insurgents on the “red cell,” a team composed mostly of U.S. veterans and others who use grenades, IEDs, AK-47s, and clever, shifting tactics to hunt squads of U.S. Marines.

The red-cell team in the Eastern bloc scenario is led by a former master sergeant with significant combat experience who tailors his attack strategies to the types of threats he expects young Marines to face while deployed. In this case, red-cell players wore civilian clothing, used distractions such as prostitutes and a medical emergency, and employed some of the insurgent tactics that non-uniformed Russian forces use in eastern Ukraine.

“We could game this and kill all [the training Marines] every time, but they wouldn’t get as much out of it that way,” said retired Master Sgt. Mark Hardin, the lead subject-matter expert. “You’ve got a 19-year-old who comes in here and needs a better understanding. So you don’t want to just overwhelm him—you’ve got to let him work through problems and take multiple runs if necessary. Mistakes are instructive.”

And every so often, Hardin says with pride, the Marines he’s training find a way to “put a hurtin’ on me.”

“We rarely face force-on-force combat anymore,” Hardin said of the Marine Corps. “It’s mostly counter-insurgency now, and there are different types of insurgencies, such as a failed state that you have to stabilize until a snap government is put in place. So we take all those possible scenarios on in here, and there is no fairy dust. We make it feel as real for them as possible, and they have to work through the problem-solving.”

All while being screamed at, and solicited, in Russian.