Edouard Elias first embedded with the French Foreign Legion in the summer of 2014 during France’s military intervention in Central African Republic. At first, the legionnaires were wary of yet another journalist shadowing them. They believed that most journalists were only interested in their own work, says Elias, and not in the mission of the troops—but the French photographer, who had been kidnapped in Syria in 2013 and spent 10 months in captivity, proved otherwise when the Legion ran into trouble.

“One day, there were some skirmishes,” he remembers. “They saw that they could count on me in the field and that’s when they really accepted me.”

Elias had spent several months in Syria confined to an underground cell with James Foley and other journalists. So, when the news of Foley’s execution at the hands of ISIS reached the regiment, the legionnaires were prepared. “They learned about his death before I did, so they were all around me when I heard the news, beers at the ready,” he says. “It sounds stupid, but that counts. They became friends, in a way.”

See the French Foreign Legion in Central African Republic

After that first embed, which lasted just a few weeks, Elias was invited in 2014 to the regiment’s barracks in Nimes, France, to celebrate—and photograph—Christmas. And just a few months later, he embarked on a month-long training exercise with the troops. “It was the biggest mistake I’ve ever made,” he says, laughing. The training included long and grueling nocturnal walks, including a 40-kilometer one that began with an eight-kilometer run. Over a month, he lost 10 pounds. “To gain their respect you have to show that you’re suffering as much as they are, that you’re doing it all,” he says.

During the training, Elias saw how rigorous and, at times, authoritarian the French Foreign Legion can be. “They’ve developed a cult of tradition and of rules,” he says. “It goes beyond anything I’ve seen before. It’s interesting to see how we teach these young men to become one being, to become the French Legion. They do this through an intense training program, but also by limiting what they can do when they’re outside (for example, they’re not allowed to leave the barracks in civilian clothes). It was unbelievable.”

But Elias cherished the experience, building relationships, he says, that will last his lifetime. “I will continue to follow them. If they’re deployed, once again, overseas, I’ll want to go with them. If one of them leaves the Legion to open a bed and breakfast in Patagonia, I’ll go. These are stories of men who shared something important and I want to see how it evolves. It’s going to be a life-long project.”

Edouard Elias is a French photographer represented by Reportage by Getty Images.

Olivier Laurent is the editor of TIME LightBox. Follow him on Twitter and Instagram @olivierclaurent