Some 15,000 rain-drenched soldiers and sailors cheered as Gary Cooper took the stage in Port Moresby, Papua New Guinea, on the first leg of a 24,000-mile USO tour in 1943. Cooper, then Hollywood’s biggest star, couldn’t sing or dance, so he launched into a monologue of jokes that his pal Jack Benny had sent him. But halfway through the show, a voice cried out, “Hey, Coop! How about Lou Gehrig’s farewell speech to the Yankees?”

****HANDOUT IMAGE “The Pride of the Yankees,” by Richard Sandomir, (credit: Hachette) ***NOT FOR RESALE (Hachette)

It had been nearly 18 months since Cooper, starring as Gehrig, had performed the brief speech on a Hollywood soundstage for the biopic “The Pride of the Yankees.” But after taking a few minutes to jot down the words, he slipped back into the role of the famous doomed athlete, whose golden baseball career was struck down by amyotrophic lateral sclerosis, a fatal neuromuscular disease.

“People all say that I’ve had a bad break,” Cooper concluded. “But today — today — I consider myself the luckiest man on the face of the earth.” The troops burst into applause. And Cooper wound up giving the speech at every stop on the tour.

That touching moment captures the main point of Richard Sandomir’s spirited, no-frills account of Gehrig’s tragic demise and the Hollywood movie depicting it. In the minds of millions of Americans — most of whom in those pre-television days had never seen Gehrig play — Gary Cooper had become Lou Gehrig. And the movie helped turn Gehrig’s story into an American folk legend of courage and dignity that young men facing combat were keen to hear.

Gehrig’s story has been told many times, but it remains a compelling tale. And Sandomir is smart to give the movie equal time in his narrative. His focus ultimately isn’t on the real Gehrig but on the myth that the filmmakers, aided by Gehrig’s fiercely determined widow, set out to create.

Gehrig helped lead the Yankees to six World Series victories and still tops the list of greatest first basemen in baseball history. Perhaps his biggest achievement was the record he set for most consecutive games — 2,130 over 14 seasons — finally surpassed by Baltimore’s Cal Ripken in 1995.

Sandomir’s account jumps right into the 1939 season when Gehrig’s swift physical decline sent him permanently to the bench on May 2. Six weeks later, he got the verdict of ALS from the Mayo Clinic. It was, wrote a reporter, “a death warrant in his pocket.” On July 4, he made his last public appearance in a Yankee uniform between games of a doubleheader where he haltingly delivered his gracious farewell. Two years later, at age 37, he died.

Sandomir, a longtime sports and media reporter for the New York Times, has a good eye for compelling characters. Chief among them was Lou’s widow, Eleanor, a passionate, sharp-tongued guardian of his legacy. When she met him at a party in her home town of Chicago, she found him a charmingly shy and unworldly urban bumpkin. “Lou and Eleanor’s union was one of an introvert and extrovert,” writes Sandomir. They were “a wallflower and a party girl; a poor boy and a girl whose family knew wealth for a time but lost it.”

Author Richard Sandomir (Terri Ann Glynn)

Eleanor nursed Lou through his devastating final days — he couldn’t feed or wash himself and lost 60 pounds as his muscular frame withered to rag-doll limpness — then hired a fast-talking New York agent to negotiate a film deal. They signed for $30,000 with Samuel Goldwyn, a famously arrogant and independent studio magnate, who promised Mrs. Gehrig veto power over the script.

“Based on a true story” is Hollywood’s favorite euphemism for “The following movie is mostly fiction.” And “Pride” was no exception. Goldwyn and his screenwriters turned Eleanor into a spunky but bland ingenue and toned down the conflicts between her and Lou’s domineering mother.

To give the film an air of authenticity, Goldwyn hired 47-year-old Babe Ruth, who lost 50 pounds and dyed his hair black to look more like the slugger who had terrorized American League pitchers alongside Gehrig during the golden days of “Murderers’ Row.”

But Goldwyn had no intention of making a baseball movie. Too boring, he said. Instead, he wanted a lachrymose paean to a great American hero. And he hired Damon Runyon to write a prologue connecting the film to the war effort. Gehrig’s story, wrote Runyon, was “a lesson in simplicity and modesty to the youth of America. He faced death with the same valor and fortitude that has been displayed by thousands of young Americans on far-flung fields of battle.”

The lean, lanky Cooper, who hailed from Helena, Mont., and had never played baseball a day in his life, became the broad-chested, steel-thighed Iron Horse with a thick Noo Yawk accent.

It didn’t matter. Cooper was himself a legend: a performer with a minimalist, aw-shucks manner and matinee idol looks that made him a celluloid natural. “The grand thing about Cooper is that you believe everything that he says or does,” said famed director Howard Hawks.

To play Eleanor, Goldwyn hired Teresa Wright, a willowy 23-year-old actress with a wide, innocent smile. She was nearly 20 years younger and a foot shorter than Cooper, but she was no pushover, as her list of contract conditions made clear: “I will not pose for publicity photographs in a bathing suit. . . . I will not be photographed on the beach with my hair flying in the wind, holding aloft a beach ball. . . . I will not be shown happily whipping up a meal for a huge family.”

Cooper had to learn baseball from scratch, coached by Lefty O’Doul, a former All-Star. “You throw a ball like an old woman tossing a hot biscuit,” O’Doul informed him. After six weeks of training, Cooper managed to look authentic, helped enormously by having former Brooklyn Dodgers star Babe Herman serve as his movie double.

“Pride” opened in New York City on July 15, 1942 — just 13 months after Gehrig’s death — to long lines and warm reviews. Variety called it a “stirring epitaph.”

Truth be told, it’s hardly a classic, despite Sandomir’s wishful subtitle. Although the acting is uniformly excellent, the love story is soggy, the humor predictable and the direction by Hollywood veteran Sam Wood utterly cliched. But Cooper’s performance soars over the last 10 minutes as Gehrig’s body begins to fall apart. While his outward demeanor remains stoic, his eyes grow wide and slightly wild, and his work gains power and pathos.

For Gehrig’s final speech, Cooper walks slowly to the microphone, shoulders slumped, eyes moistening. He runs his hand through his hair and talks haltingly — an inarticulate man somehow finding the words for his own eulogy.

The transformation was complete. Lou Gehrig was dead, but thanks in large part to Hollywood, his legend is eternal.

Glenn Frankel's latest book is "High Noon: The Hollywood Blacklist and the Making of an American Classic."

The Pride of the Yankees

By Richard Sandomir

304 pp. $27