Ten years ago, Steve Erzinger was an undersized senior linebacker at 210 pounds and one of the captains of the Army football team. Yet when he headed into his final game against Navy, he weighed barely 190 pounds.

Hours before kickoff, Erzinger was on a training table in the bowels of FedEx Field in Maryland, hooked up to an I.V. as fluids coursed through his body. He had the flu, but most of his weight loss had come before the illness.

He was not alone among the Black Knights. I had watched one player after another shrink over the course of the 2011 season as they tried to balance being soldiers and students and athletes. Nagging injuries became chronic ones, because the cadets still had to attend to their military duties, go to class and the library while participating in their sport.

There was no rest or recovery. There also was no way Erzinger or any other of the bruised and battered players in either the Army or Navy locker rooms were going to miss (pick one) America’s Game, the Civil War or the Game of Honor.

American service personnel across the world, veterans and the college football faithful will watch and celebrate this ethos when Army (8-3) meets Navy (3-8) on Saturday for their 122nd matchup, this time at MetLife Stadium in East Rutherford, N.J.

But in this new N.C.A.A. name, image and likeness era, the service academies are home to Division I’s last true student-athletes whose play largely comes secondary to their other pursuits. Because the government pays for their tuition, housing and fees, Cadets and Midshipmen are considered employees and federal laws forbid the use of public office for private gain.

“I’m biased, but the service academies have always been the cornerstone of what a student-athlete should be,” Erzinger said. “We have a mandatory curriculum and military duties that could not be avoided. Don’t get me wrong — athletes at schools with different work-life balance deserve their share of the money. The N.C.A.A. has always been a money game, but we are not them.”

The N.C.A.A., facing pressure from numerous states, changed its rules this year to allow athletes across its three divisions to seek outside deals, including endorsements and other forms of income. Still, the association’s shifting stance has underscored a clear red line for college sports administrators: unlike the service academies, universities at large do not want athletes to be considered employees.

At West Point, N.Y., where I spent a year researching for a book, cadets take 17 to 20 hours of Ivy League-quality classes and participate in year-round physical and tactical training to maintain the discipline the military demands. No summer vacations, or much opportunity to shift a course to the summer to lighten the academic load during the season.

Beast Barracks — or basic training — begins in late June before the freshmen start classes. Upperclassmen undergo leadership training, which can include simulated combat missions and Ranger School and can take cadets to places like Fort Benning, Ga., and Germany.

Playing football is at once the easiest, most fun and least important thing they do over the course of their 47 months as officers in training.

In the modern era, a handful of them have achieved careers in the N.F.L. Baltimore Ravens offensive lineman Alejandro Villanueva, a two-time Pro Bowler, played for Army. New England Patriots long snapper Joe Cardona, who played for Navy, has won two Super Bowls.

The vast majority end up serving their country for a minimum five years. Erzinger, for example, qualified as a Ranger and deployed with them to Afghanistan. He made captain as a member of the 173rd Airborne Brigade, serving in Estonia, Lithuania and Ukraine.

He left the Army in 2017, got an M.B.A. at Rice University in Houston and is an investment banker in the energy sector there. He is married and has a 16-month old son, Eli.

Another of his football co-captains, Capt. Andrew Rodriguez, commanded the 1st Stryker Brigade Combat Team of the 4th Infantry Division, earned a master’s in mechanical engineering and business at Massachusetts Institute of Technology and teaches at West Point.

Among his team’s expansive text chain are Green Berets and educators, bankers and engineers, small business owners and real estate developers. The thread blows up this time of year, with talk of lessons learned at West Point and camaraderie that is missed.

“What I learned from football and West Point is how to endure pressure and figure out what works. If everyone gets behind a mission, you succeed,” Erzinger said. “I had a sense of purpose in the Army. You go to the outside world and it’s more an individual sport.”

There are memories of victories as well, though precious few of them. Erzinger had only one winning season, in 2010, when the Black Knights defeated Southern Methodist University in the Armed Forces Bowl.

He also never beat Navy.

In his senior season, Erzinger came off the training table and I.V. drip and led a swarming defense to the brink of an Army victory. The Black Knights were down 27-21 on the Midshipmen 25-yard line with a little more than four minutes remaining. It was fourth-and-7. They didn’t get it.

At that moment, he was devastated.

“Almost doesn’t work,” he said, his eyes red. “It’s something I have to live with now.”

A decade later, it does not hurt so much. He planned to barbecue at his home on Saturday with about a half dozen other West Pointers — including some teammates — and their families.

“I want us to win,” he said. “But mainly I want it to be competitive and for both teams to come out of healthy. I know where players from both teams have come from and where they are going. We all made a commitment and I do not regret mine. I’m sure they won’t either.”