When U.S. Army Ranger Keni Thomas was writing to his mother on a day off in Somalia, he had no idea he'd soon be part of a devastating moment in American military history: the shooting down of two Black Hawk helicopters in Mogadishu.

Thomas has given his powerful account of the 1993 incident, which has inspired a best-selling book and feature film, for "The Real Black Hawk Down" episode of National Geographic Channel's new "No Man Left Behind" series. 

Premiering on June 28 at 9/8c, the series combines gripping personal testimony with vivid reenactments and archival footage to shed light on the stories of modern war heroes and special agents who overcame incredible odds in hostile environments.

In August 1993, Thomas was deployed to Somalia as part of a special operation intended to capture Somalian warlord Mohamed Farrah Aidid, who had been attacking the United Nations food shipments and killing American troops.  

"It was a manhunt," Thomas recalled. "We had to start getting his upper echelon of people…. We weren't part of a peacekeeping force; we weren't there to feed the hungry; we were there to kick in doors and root this guy out."

After participating in raids that captured some of Aidid's militiamen, Thomas decided to write a letter home on Sunday, Oct. 3. "I never finished the letter,” he said. “I was lying to her, because it said, 'Dear Mom, You would love it here in sunny Mogadishu.' "

Suddenly, Thomas and the other rangers heard the call to "get in on" a rare daytime raid.

It started out like any other mission. The rangers descended from the helicopters by rope to follow the Delta Force assault team after it had captured some of Aidid's militiamen in a building.

They finished the raid in 30 minutes. The prisoners had been loaded onto trucks and the American military men were waiting for a ride back to their base — when all hell broke loose.

"The first helicopter got shot down…. When the radio's [saying], 'There's a Black Hawk going down,' everybody kind of looks up. At first, you don't want to believe it…. Once you figure it out, that's what's happening, you gotta do something about it.

"The mission changed into a rescue. We've got to get to that crash and hope there's people still alive at the crash and we can get there before the bad guys," Thomas said. "Because you could see…in the cross streets…on the side alleys, just people all sprinting to where the crash went down."

When Thomas' squad leader was wounded, his platoon sergeant put him in charge.

Thomas and his fellow rangers fought on the narrow streets, with gunfire everywhere. Thomas' magazine pouch got shot, but he wasn't injured. It turned into an 18-hour battle. "We just didn't count on that many people [against us],” Thomas said. “It was a 10-to-1 ratio…. It was pretty chaotic.

“There was a lot of gunfire … when people start getting hurt and shot … it's very real…. If not your friend, it's someone you counted on, and then you've gotta do something to fill the gap, and the bad guys keep coming and you can't call a timeout."

Later, Thomas said, they were helped by American 10th Mountain Division forces and Malaysian and Pakistani military; trucks finally were able to pull up and transport the wounded away.

But there was no room left for the uninjured men. They had to sprint to a stadium for safety.

"There was only one time where I could feel that fear in me,” Thomas said, “and it was on the run out of the city … when there's only 25 of us running and there's no help, there's no tanks, there's no air support, there's … just you running on the ground.… The emotion of that moment starts clouding your better judgment…. Guys were running too fast, and then it was too spread out…."

A mile from the stadium, a Malaysian armored personnel carrier pulled up, and Thomas and others got in with "rounds hitting the side of it. All you can do is sit there and imagine, 'Oh, crap, someone's going to shoot…and this thing's going to explode.' "

Thomas survived, but others weren't so lucky. Eighteen Americans died and 73 were wounded in the Black Hawk Down battle.

Today, Thomas tells his story as a motivational speaker and works as a country music singer.  

He returned to Mogadishu a few years ago and marveled, "There were still bullet holes in the same walls 20 years later…. I said, 'How did any of us get out of here?'

"I've learned the value of how important it is to take care of the people around you."

American Legends – Stories of soldiers who responded to a difficult conflict situation, and did everything they could to defeat the enemy and get their brothers home.

"No Man Left Behind" premieres Tuesday, June 28 at 9/8c on the National Geographic Channel.

For more information and clips, visit natgeotv.com/NoManLeftBehind.