Josh Hall punched the ball from Williams' arms just as he was twisting his way into the end zone in the waning seconds of the first half and the ball bounced away from a teammate for a touchback instead of a touchdown.
Rather than a three-touchdown cushion at halftime, the Black Knights went into the locker room dejected over a slew of mistakes and missed opportunities that made their 14-0 lead feel tenuous.
"We had a chance to be in the end zone twice more and managed not to do it," Army coach Rich Ellerson lamented. "Then in the second half, it was a tale of turnovers and penalties."
The fired-up Falcons rallied in the second half for a 24-14 win that allowed them to retain the coveted trophy that signifies superiority among the service academies.
Williams' fumble was the beginning of Army's downfall.
"I think it was the start of the turnaround," Hall said, "because as a defense we played amazingly the second half. We were having a horrible first half. I felt like it could have been 35-0 easily."
Conversely, the Black Knights headed into halftime kicking themselves.
"We could have gone into the half 28-0, and if we'd have done that, the second half probably would have gone a different way," Army fullback Jared Hassin said. "We need to find that fire in the second half."
After being outgained on the ground by 230 yards in the first half, the Falcons (5-4) outscored the Black Knights 21-0 in a third-quarter blitz that included two field goals, two touchdowns, a two-point conversion and two takeaways.
Parker Herrington kicked a career-high three field goals for the Falcons, who recovered fumbles that allowed them to start drives at the Army 29, 19 and 14 in the second half.
Until its meltdown, Army (3-6) looked primed to end its 11-game skid against the service academies and win a share of its first Commander trophy since 1996.
The Falcons were fortunate to only be trailing 14-0 at halftime. Air Force stopped Army on fourth down twice in the red zone. The Black Knights were denied once at the 1 to go with Williams' fumble through the back of the end zone.
Air Force started the second half with a drive that ended in Herrington's 37-yard field goal. After a punt, Jefferson's 2-yard keeper and Asher Clark's 2-point conversion run pulled Air Force to 14-11.
Army quarterback Max Jenkins fumbled the snap on the ensuing drive and that led to Herrington's 39-yard field goal that tied the score at 14.
The Black Knights then bungled a fake punt, and linebacker Stephan Atrice scooped up the ball at the Army 14. Four plays later, Jefferson's 1-yard TD run gave Air Force a 21-14 lead.
"I love the guts they had to fake that punt," Falcons coach Troy Calhoun said.
Early in the fourth quarter, Jon Davis stripped Hassin and returned the fumble to the Army 19. Herrington followed with a 39-yarder that capped the comeback.
Davis, who tied his career high with a dozen tackles, sealed the win with an interception deep in Air Force territory with four minutes left, sending Army to its sixth straight loss to the Falcons.
Air Force beat Navy 35-34 in overtime last month and won its second straight Commander-in-Chief's Trophy after ending its eight-year drought.
Last season, Jefferson, who finished 6-1 against Army and Navy, was bummed to learn only the seniors got to go to the White House to meet President Obama.
The Black Knights knew that if they had capitalized on more of their first-half chances, they might be the ones making plans for a visit to the Oval Office in a few months.
"It certainly isn't the outcome I wanted or hoped for," Hassin said, "but at the same time, this is a great rivalry and I couldn't be any happier for my future brothers that I'm going to be serving with. They're a tremendous group of guys, and they've got a great coach and a great leader. I couldn't be happier for them."
Army mistakes lead to loss to AFA, 24-14
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