Corporal Sterling is a soldier dedicated to the NCR. He was, at one point, a Ranger, and involved in the Hoover Dam Scouting campaign of '73 (prior to the first battle to take place in '77), but has since fallen on hard times. While patrolling the Mojave Wastes near Malpais, he was captured by Legion slavers.
Under their brutal incarceration, he was viciously tortured and maimed, with his captors severely mutilating both his hands and feet. After they were done torturing him they left without tying him, figuring he wouldn't be able to escape. He then crawled out on his elbows and knees to the edge of the camp and rolled down the hill into the Colorado River, where he was then rescued by some observing rangers.
Unable to continue working due to his grievous injuries, he was discharged. Lieutenant Gorobets requested that Sterling be transferred to 1st Recon, believing that while he may not be able to do the trekking required for a Ranger any longer, he can still be an asset to the NCR as a sharpshooter.
He mentions he was captured near Malpais. This might be El Malpais National Park in New Mexico. But this is rather out of the way, and surely the NCR would have found Hoover Dam by then if he made it out that far, rather than a year later.
Oddly, when the quest I Put a Spell on You is active, and after completing Three-Card Bounty, speaking with Corporal Sterling will provide a dialogue option about the lights at the Camp McCarran Control Tower, even though at that point he will have relocated to Camp Forlorn Hope.
"Used to be a Ranger... one of the first they sent out east, back before we took the Dam. Observation and reconnaissance. We took the lay of the land, checked out the locals, and kept ourselves inconspicuous. Couple friends of mine were the first to scout the Dam. That was back in '73, if I recall. Lot of those Rangers are dead now. Vegas always chewed men up... it's just a little more literal, nowadays."
"Well... that wasn't really a matter of choice. Got myself caught by Legionnaires, up near Malpais. They had themselves some fun with me... mangled my hands and feet pretty good. Wasn't much good with a pistol after that. Wasn't gonna be trekking across the waste on any more long scouts, either."
There is a cell found within the GECK, above the eastern cliffs of the Colorado River and across the river from Camp Forlorn Hope, named "MalpaisLegionCamp," perhaps alluding to Sterling's capture by the Legion near Malpais.