Every single holiday season, “It’s a Wonderful Life” warms countless hearts. The 1946 film starring Jimmy Stewart is still an unmatched classic whose popularity is unshaken even in today’s complex cinema landscape, with its countless digital extravaganzas and giant movie budgets.

A new book, “Mission: Jimmy Stewart and the Fight for Europe” by Robert Matzen reveals some surprising truths behind the film and its leading man.

“Even in his time, in the 1940s, Stewart was unique.”

With the 70th anniversary of the film coming up on Christmas Day this year, it’s the perfect time to find out about the one-of-a-kind movie star who took Hollywood by storm with roles in classics such as “Vertigo” and “Mr. Smith Goes to Washington.”

The piece of Stewart’s life most fans don’t know about — and which he did a very good job of hiding — is his military service. Stewart (who died in 1997) left behind the glitz and glamour of Hollywood to serve in active combat as a bomber pilot in World War II. He even eventually rose to the rank of brigadier general, arguably matching his Hollywood success with his valiant military service.

But returning from war was difficult for Stewart. “He could no longer play youthful, leading man roles,” Matzen told LifeZette. Stewart had aged badly from combat and was suffering from post-traumatic stress disorder — something the world knew very little about at the time.

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Stewart’s Hollywood comeback arrived with “It’s a Wonderful Life,” a film the new book focuses on and which was integral to Stewart’s reintroduction to civilian life. “Life” matched Stewart with director Frank Capra, another military veteran who had the same hopes of a comeback in Tinseltown.

“Neither of them had any of options when they came back to Hollywood but to work with each other,” said Matzen, who is based in Pittsburgh and has written other books about Hollywood figures and their history.

The “Life” shoot was described by many to be troubled, as Stewart and Capra worked through the struggles with returning home and pressures the picture put on them for their careers.

“These guys were so on edge and they were so tense. So much time had passed and they feared they were no longer relevant,” said Matzen. Stewart, in Matzen’s words, had to “recreate his career on the fly.”

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Though Stewart’s infamous decision to leave Hollywood for a uniform is well-known, the details of his service have always been scarce. “Stewart would not talk about his experiences and that stopped every writer from going in-depth on it,” said Matzen — who did not let the lack of public details deter him. “It was my research challenge.”

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He continued, “I found guys who had flown with him and then I started to uncover this tremendous story.”

The “tremendous” story is one that seems like it could not come out of today’s Hollywood. To drop a successful Hollywood career to serve one’s country is the sort of valiance appropriate to America’s greatest generation.

When asked whether years like 2016 can produce men like Stewart, Matzen said, “It is possible.” He cited the late football player Pat Tillman, who dropped a career in the NFL in 2002 to serve with the Army Rangers in Afghanistan and Iraq. “He served in pretty much the same way,” said Matzen.

The author is adamant that Stewart is one of a kind, however. “Even in his time, in the 1940s, Stewart was unique,” said Matzen. He added, “It’s so different with Stewart because of that family mission.”

The book goes into depth about Stewart’s father and grandfather, veterans themselves who would inspire Stewart.

Before Hollywood, before acting, Stewart watched his father ship off to the Great War in 1917. A picture in “Mission” shows a nine-year-old Stewart in a photo with his father before he’s sent off wearing a matching military uniform.

Said Matzen, “He’s already dreaming of being in the service.”