Any biographer of a great leader must address this question: What happened before the greatness that helped the leader become worthy of so much marble? In his new book, Becoming F.D.R.: The Personal Crisis That Made a President, Jonathan Darman argues persuasively that there would not be the Franklin Delano Roosevelt we revere today if, in 1921, 12 years before he became president, he had not been stricken with polio, a disease without a cure that, if it did not kill, could easily break a man’s spirit. How Roosevelt, his wife, and his aides did not allow that to happen is Darman’s riveting tale, filled with the drama, detail, and insight that is the hallmark of a first-rate historian.

JIM KELLY: My guess is that when you started working on Becoming F.D.R., you did not think that polio would re-emerge this summer as a singular threat, with one unvaccinated man in New York State already paralyzed, the virus found in New York City wastewater, and London offering booster shots to any child under 10. For most folks, polio is a distant and nearly extinct disease, but when Roosevelt at age 39 contracted infantile paralysis in 1921 it easily could have been a death sentence. How much did you have to immerse yourself in learning about the disease?

JONATHAN DARMAN: In the space of a couple of summer days, Roosevelt went from someone who appeared healthy—running around, sailing, and swimming with his kids—to someone who was paralyzed and could not control his muscles or bodily functions. He lay in fevered agony for two weeks before he was correctly diagnosed with polio.

The most important thing to get my head around was the combination of mystery and terror that would have come with that diagnosis in 1921. By then, polio was a source of universal fear—2,000 New Yorkers had been killed in a single outbreak a few years before. But doctors and public-health officials treating polio patients were still in many ways in the dark. Most were not trained to spot the early signs of infection. They still didn’t know how it was communicated. Some speculated it was insect bites. We now know poliovirus lodges in the throat and intestine after oral contact with trace amounts of fecal matter. And we also now know that many, many people got polio as a mere digestive ailment, experiencing only mild or moderate symptoms. Roosevelt was among the unlucky minority for whom it was a life-altering event.

J.K.: You do a masterful job in cutting back and forth across Roosevelt’s life before he became president, reminding readers how virile and strong he was as the Democratic Party’s vice-presidential candidate in 1920 (sharing the ticket with Ohio governor James Cox) and that, though the Republicans won, Roosevelt was still seen as very much a comer in politics. By this time, his wife, Eleanor, was already aware of Roosevelt’s romance with Lucy Mercer, and yet once the disease struck him down, she devoted herself utterly to his recovery. What do you think bound her so to him?

Roosevelt swimming in Warm Springs, 1929.

J.D.: After the Lucy Mercer affair, Eleanor had resolved to have an independent life, still married to Roosevelt but apart from him. Then he got sick, and it seemed to wrench her back into his orbit. She didn’t complain. Up to that point, Eleanor had had an unhappy life; I don’t think she fully believed that pleasure or even contentment were things she had a right to expect from the world. So, a life spent caring for a husband who had betrayed and humiliated her probably made sense to her.

What she couldn’t see was that Roosevelt’s polio would soon prove the great accelerator of her independence—she went out into the world to represent the Roosevelts in politics while he focused on his rehabilitation—and that her independence would ultimately bring her not just a life of significance but also a life of joy.

J.K.: Roosevelt’s sudden sickness could not be hidden from the press, but what those in his camp did so well was to make it seem that he had recovered far better than he actually had. Roosevelt, in fact, seemed optimistic that he would regain more movement in his legs than he ever did. Do you think there was a point in time that he realized, in fact, that he would always require a wheelchair?

J.D.: For seven years after his polio infection, Roosevelt devoted much of his time to trying to regain the ability to walk unaided. After a couple of years, his doctors and physical therapists could see that it was not physically possible. But Roosevelt still believed he could do it.

There’s a dramatic moment in the autumn of 1928 when you can see him give up his dream once and for all. Al Smith is putting enormous pressure on Roosevelt to run as the Democratic candidate for governor of New York. Roosevelt resists at first, because he still believes, against the evidence, that if he can just have a couple more years devoted to rehabilitation—not politics—he can regain the ability to walk. Eventually, he gives in to Smith’s pressure and gets into the governor’s race, a choice he knows could put him on a path to the White House. But he also feels he is trading away his chances of ever walking again.

J.K.: I did not realize how much the disease had sapped Roosevelt’s wealth, and how much he had to scrape together to buy Warm Springs, a decrepit resort in Georgia whose mountain-fed waters promised curative powers. Provisions were so hard to come by that Eleanor would watch the cook chase a chicken around the yard, kill it, and then serve it for dinner that night. Yet Roosevelt welcomed not just the energizing effect of the pool’s water on his legs but also took great pleasure in seeing how other patients found solace there. He obviously drew mental and emotional strength from them. He even had a special section reserved for some of them at his first swearing-in as president. Any patient that you think was particularly inspiring to him?

Roosevelt on vacation in Warm Springs.

J.D.: What inspired him most was observing progress in Warm Springs patients. In my book, I quote from a patient’s report from the mid-1920s; it has detailed descriptions of how the patient’s mobility has improved, which muscle groups have strengthened, etc. At the bottom, the report is signed by its author: Franklin D. Roosevelt!

There’s something poignant in the pleasure Roosevelt took in Warm Springs patients’ progress, because he was largely failing to make the same kind of progress himself. That’s why Warm Springs is so key to understanding Roosevelt’s presidency: it’s the first place he really devoted his life to serving others, and also the place where he discovered his unique ability to foster recovery and inspire hope, the very things that would make him an ideal leader during the Depression and the war.

J.K.: During much of the time Roosevelt was convalescing, in the 1920s, Eleanor took on a much more public role around the country and found herself happier and more engaged with the world than before. You argue quite persuasively that the illness not only made Roosevelt a more serious and more patient man—without damaging his first-rate temperament—but that it changed Eleanor for the better. How so? And do you think the illness saved their marriage?

J.D.: Without polio, the Roosevelts might have stayed married, but they never would have found an easy way to coexist. Roosevelt’s disability awakened him to how much he needed Eleanor and all that she could offer him as a political partner. And it pushed Eleanor out into the big world, the place she was always meant to be.

Her old rage toward Roosevelt from the Lucy Mercer days never fully went away. In my book you see it surface occasionally, like the day after he won the New York governor’s race, when she told reporters her husband was “bound to get fat” because he wouldn’t have as much time for exercise. But she appreciated the ways that illness and recovery deepened his character. She and Roosevelt were able to form a new union based on their shared ability to remake themselves in the middle of life.

First Lady Eleanor Roosevelt visits the indigent remnant of the Bonus Army, in Fort Hunt, Virginia, 1933.

J.K.: There have been so many multi-volume biographies of Roosevelt by some of our finest historians that I wonder if you have a favorite? And given the fact that your previous book delved so much into the personalities of Ronald Reagan and Lyndon Johnson, why do you think you are so drawn to the personal lives of politicians instead of, say, inventors?

J.D.: Not a multi-volume biography, but Robert E. Sherwood’s classic Roosevelt and Hopkins is just a marvelously rich book. Sherwood wrote about Roosevelt’s “heavily forested interior.” I think most people who write about Roosevelt probably have a period where they feel they’ve gotten hopelessly lost in that forest.

I grew up in Washington, and I’ve always been fascinated by the unintended consequences of the power game: the way it affects public figures’ inner lives, the ways it gets absorbed in their families and in their other relationships. The lives of politicians can be wonderful windows into human nature. I know that sounds absurd—no one is going to mistake Marco Rubio for Michel de Montaigne. But they live meticulously documented, closely observed existences, and they are put into the most extraordinary circumstances. So, researching a politician’s life is sort of like watching a lab experiment on the emotional mechanism.

J.K.: Both of your books are so rich in detail and anecdote, I wonder what you like doing the most, the research or the writing. And is there a particular nugget or two that you have dug up that made you so happy you shouted, “Eureka!”?

J.D.: Oh, the research is the fun part! You stumble on a unique voice in a letter, and it can reveal your whole book to you. This book really came together for me when I read Roosevelt’s correspondence with other polio patients. Many of these people were strangers, but Roosevelt was often more frank and open about his condition with them than he was with some of his closest intimates.

Roosevelt visits the Orthopedic Hospital in Seattle during his 1932 presidential campaign.

Some polio survivors wrote seeking advice about the treatments and rehabilitative approaches he had pursued. Others wanted to offer advice themselves. One man, a working-class polio survivor from Massachusetts, described how pride, anger, and fear had impeded his own rehabilitation after polio. “Mr. Roosevelt,” he wrote, “don’t worry, whatever you do. It won’t help any.” When I read those words, I saw a straight line to “The only thing we have to fear is fear itself.”

J.K.: What drew you to journalism in the first place? You are so remarkably deft at bringing to life historic moments and capturing the tension inherent in the best narratives that I wonder if you have ever considered writing a novel or screenplay. And if not, are there any fictional works that have inspired you in storytelling?

J.D.: I think as a Very Serious Writer of History I’m supposed to say that the truth is always more interesting than fiction. And maybe that’s true, though I suspect I’d find the world a more interesting place if I began each day reading Austen or Dickens instead of what I actually do—scrolling through Twitter and ingesting whatever Trump horror story is leading the New York Times app.

I like narrative nonfiction because of the structural constraints it imposes. I learned in my magazine-writing days that it’s much easier to stare down a blank page when you’re dealing with a good set of immutable facts. You know there are many ways to assemble those facts into a story—but not an infinite amount. Novelists don’t have that built-in limitation; I admire their ability to impose discipline on their own imaginations. Edith Wharton and Anthony Trollope are indispensable guides for writing about people. They both remind you that insight and social observation are things you can provide in passing, but your main task is moving the story along.

To hear Jim Kelly reveal more about his story, listen to him on AIR MAIL’s Morning Meeting podcast

Becoming F.D.R.: The Personal Crisis That Made a President, by Jonathan Darman, will be published on September 6 by Random House

Jim Kelly is the Books Editor for AIR MAIL