When Army beat Air Force, 21-0, in November 2017, a plot against the third member of the Commander-in-Chief’s Trophy was buried in Army’s play calling. As the Black Knights shut out the Falcons in the second half, Army ran a play out of a particular look.

It’s one that looks like it’s meant for the split zone, a play common in more than just triple option offenses. It’s basically when a blocking H-back moves across the line during a normal zone run. It’s used by lots of teams in pro and college, such as Notre Dame and Ohio State.

Against Air Force, Army ran plenty of shotgun plays with the backs bunched around the QB in a diamond.

But they moved one back to the line and showed this specific look once, a tendency breaker that would jump off the screen for an opposing coach:

Army started running out of this during the Navy game, at various downs and distances, against a variety of defensive fronts, at different times of the game, etc. If Navy noticed the shift while viewing the Air Force film, they definitely noticed how often the formation was deployed in Army-Navy. Watch how many defenders have crept toward the line:

“That’s when we knew it was worth it, that it would work,” Army offensive coordinator Brent Davis said. “We saw their safeties and knew a pass would work, maybe even to score, and we had to decide when to try it.”

Finally, Army lined up against Navy in its split zone look on third down from its own 23 in the third quarter. No. 85 Quinten Parker stepped forward from the H-back spot to indicate a run, as Army had put on film against Air Force, drawing Navy’s safeties up to stop the ball carrier.

Except this time, Calen Holt shot post the creeping secondary and caught a wobbly pass from Ahmad Bradshaw in a driving snowstorm, Army’s only pass the entire game.

It was 20 yards that kept Army alive, down 13-7, and maintained field position. The Knights would miss a field goal from the Navy 18 on that drive, but force a three-and-out inside Navy territory and score the winning touchdown on their next possession, sweeping their rivals on the year. This play helped decide one of football’s biggest rivalries, and it was set up over the course of an entire season.

While coaches talk about one-game seasons and only focusing on the opponent they’re playing that week, it’s not uncommon to show calls over the course of the season in anticipation of a big opponent.

What makes Army and Navy so different are the extremes this goes to, and the circumstances.

“I watch Navy and take notes on them pretty much every Friday,” Army defensive coordinator Jay Bateman said. “I want to see what they are doing different each week and not wait until the end of the season.”

There’s no other rivalry in sports in which programs are as open about the mentality of a “one-game season,” and at the same time, there are no two programs this similar.

Army and Navy run the same rare offense, both under head coaches Jeff Monken and Ken Niumatalolo, respectively, who each once called plays for the same flexbone option head coach, Paul Johnson. As service academies, they recruit the exact same players nationwide and operate with the exact same limitations and exceptions.

“You have to do things that are different at each level of the game because the two teams know each other so well. And you also can’t be afraid to turn it loose and not play so tight and so reserved that you don’t go for it sometimes,” Army head coach Jeff Monken told SB Nation in 2016.

What Army and Navy do — and don’t do — against Air Force, a very similar program, has a huge impact on each year’s Army-Navy game.

“We do the same thing [when it comes to intentionally putting plays on film]. It’s not like this is new, revealing stuff,” Niumatalolo told SB Nation in a 2017 interview. “Coaches will talk about holding plays or discovering something, but we didn’t just find King Tut’s tomb or anything. We’ll watch what they did against Air Force. They’ll watch what we did against Air Force.”

For both staffs, especially since Monken’s arrival in 2013, the Air Force game has become very important in preparing for each other. Both staffs know that how they defend and attack another option academy will be picked apart in December, so it becomes a game within a game. They each want to beat the Falcons without showing too much, or beat the Falcons by showing red herrings.

“The Air Force game is important because that game is one where you know going in, the opponent has a plan to stop the option,” Bateman said. “And you can get a feel for what additional offensive concepts they might have added to complement the base offensive stuff. And schematically, there are similarities between all three [teams].”

An additional wrinkle to Army-Navy is Willie Fritz’s Tulane. Navy’s AAC neighbor runs an offense that’s more Urban Meyer than Paul Johnson, but it creates looks applicable to Army-Navy. This is both a disadvantage and an opportunity for the Midshipmen. They have to commit play calls and defensive schemes to tape, but they can also build in deception they hope Army sees.

During Army-Navy weeks, offensive and defensive staffs spend more time together than usual.

If you’re trying to scheme a defense to stop an academy flexbone, it helps when you have an offensive coordinator down the hall who runs an academy flexbone, and vice versa.

“I watch Navy and take notes on them pretty much every Friday.”

“[Bateman] will come in and draw something up real fast and ask ‘How would this affect you? What play would you run If I lined up like this? How would you attack this?’ So we can run the plays ahead of time that we think they might adjust to,” Davis told SB Nation during Army-Navy week in 2016.

Another massive difference from any other rivalry is the prep time.

Army-Navy is so far after every other regular season game, it’s in everyone else’s postseason.

As a FBS independent, Army schedules as much time beforehand as possible, while the Midshipmen are obligated to play a more traditional schedule as part of the AAC.

By the time Army and Navy meet on Dec. 8, 2018, the Black Knights have had 19 days to prep for one regular-season game. Navy’s schedule allowed a week less.

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The Army-Navy game is so big that emotion is something coaches have to limit, not amplify.

“It’s probably the most physical game you’ll ever see. Standing from the sidelines it’s the most physical football I’ve ever seen,” Bateman said. “I don’t ever have to coach effort here. Ever. No. We play harder than anyone else in the country. No one plays harder than us. Put the film on. I’ll send it to you. No one drags. [Former linebacker] Andrew King would kick their ass.”

“One of our best teams, when we went 11-2, we barely beat Army that season, and that wasn’t one of Jeff’s best teams. We held on for dear life and picked the ball off in the end zone,” Niumatololo said. “It’s the emotion of the game. The games are always close.”