More than 16 years and hundreds of deployments into the war in Afghanistan, the story of the first ground troops to strike back after the Sept. 11, 2001, attacks on the U.S. is still a little-known tale.

Or, at least, it was to many members of the cast and crew who are bringing it to life in “12 Strong.”

Details of the mission first came to light in 2009, when author Doug Stanton published “Horse Soldiers,” so named for the equine transportation a team of 3rd Battalion, 5th Special Forces Group operators rode into battle against the Taliban.

On Friday, a Jerry Bruckheimer-produced epic based on the book hits theaters, with Chris Hemsworth starring as Capt. Mitch Nelson, commander of Operational Detachment Alpha 595.

“In 2010, he introduced me to this script, and I was blown away,” director Nicolai Fuglsig told Army Times in a Jan. 6 interview. “So much so that I immediately went and bought Doug Stanton’s book, which blew me even more away.”

If “Mitch Nelson” doesn’t sound familiar, that’s because it’s a pseudonym — the fictional Nelson and Chief Warrant Officer 4 Hal Spencer, played by Michael Shannon, are covers for retired Lt. Col. Mark Nutsch and CW4 Bob Pennington, who asked Stanton not to use their real names when writing the book.

Other team members’ names also were changed, though 3rd Battalion commander Lt. Col. Max Bowers and 5th Group boss Col. John Mulholland went public.

“I never, ever heard about this. I couldn’t believe when I read it, that you had these modern American soldiers riding into battle, alongside these Afghan soldiers, and then on horseback — it was so ‘Lawrence of Arabia,’ ” Fuglsig said.

Stanton began reporting for the book in 2003, reading early stories on the Afghanistan war. In an article about so-called “American Taliban” John Walker Lindh, he read a tidbit about Special Forces soldiers who had discovered him during an early battle.

“When I began to think about 9/11, and I discovered that just a very small group of United States Army Special Forces soldiers went into the country — like, that was Plan A?” he told Army Times.

A dozen Green Berets, as part of Task Force Dagger, dropped into Afghanistan and met up with ethnic Uzbek warlord Gen. Abdul Rashid Dostum, who lent them some of the horses he and his men used to get around.

The plan was to make their way to a Taliban stronghold in northern Afghanistan, as units in other parts of Afghanistan followed a similar plan to bring the regime to its knees.