Ian Kershaw cuts a gentle figure—soft-spoken, bespectacled and almost stereotypically English in his unwillingness to blow his own horn. Meeting him, it's hard to believe that he is the author of what is, so far at least, the definitive biography of Adolph Hitler. But as he talked over lunch in London recently, what emerged was a man who in his unpretentious way shares the trait of all great historians—a passionate ambition to explain.

This becomes clear in his latest work, Fateful Choices, due out from Penguin Press in May. It examines 10 crucial decisions, taken in the 18 months between May 1940 and December 1941, which served to turn several different conflicts into "one truly global conflagration." It is a somewhat of a departure for Kershaw, who has worked almost exclusively on Nazi Germany for the past 30 years: "It's not my automatic territory," he concedes, "but I always like to learn a lot of things as I write."

It's a book that's full of surprises, overturning many popular perceptions about the war. Hitler's decision to invade Russia, for example, has long been seen as an ill-judged act of hubris, which fatally overextended German forces by having them fight on two fronts. Yet Kershaw presents it as a perversely logical decision, since Hitler understood that the chief obstacle to his plans for conquering Europe was neither Britain nor the Soviet Union, but time. Inevitably, Hitler thought, any prolonged conflict would draw an increasingly strong America into the war, as proved to be the case.

Two of the book's 10 chapters focus on Roosevelt's efforts to help a beleaguered Great Britain, and Kershaw says his first foray into American history was "completely intriguing, especially discovering how Roosevelt had to teeter along this tightrope all the time." Of Roosevelt himself, he admits, "I didn't start off being a great admirer, but as I wrote the book, increasingly I became a fan." The contrast in Kershaw's book between how the two democracies of Britain and America took their key war decisions is especially noteworthy: where Churchill, in order to fight on after Dunkirk, needed the backing of his cabinet colleagues, Roosevelt had to put his energy into keeping a difficult Congress sweet. Interestingly, it was only in the United States that public opinion influenced executive decisions. Few people in 1940 Britain were even aware of the conflicts in Churchill's cabinet.

Sir Ian—he was knighted in 2002, one of only a handful of historians to receive the honor—is at pains to point out that Fateful Choices is not a "what-if" book, one of the counterfactual histories so much in vogue, but a study of an interconnected series of "whys." He is particularly good on the role that abstract concepts played in what were manifestly unabstract decisions, noting for example that the nationalist aggression uniting Germany, Japan and Italy also contained a deep fear of humiliation. Pride meant that defeat was preferable to any kind of surrender, as Hitler's own suicide and his willingness to condemn his country to the same fate attest.

Fateful Choices is at once a scholarly book, thoroughly documented and full of research, and an easy, often riveting read. Kershaw himself refuses to accept the common division between academic and popular history: "It never occurred to me that I had to be one or the other. I've just written what I wanted to write."

Though Kershaw is best known for his work on Nazi Germany, this is not the first time his work has changed course. The son of a shop owner and semiprofessional jazz musician, Kershaw received his undergraduate degree from Liverpool University before doing a Ph.D., but his early historical interests were really shaped, he recalls, "by my Catholic background. I went to Catholic grammar school, the last bastion of pure Renaissance scholasticism." His doctoral dissertation (and first book) used a recently discovered thousand-page manuscript of accounts from a medieval monastery "to explore what life was like there. At the time, I was a 100% convinced medievalist."

How, then, does he explain the move from monasteries to the 20th century's greatest monster? Kershaw answers cheerfully that he can't: "I really think the honest answer is they were two completely separate worlds I happened to be interested in." His fascination with German history began after an unsettling encounter: "In 1972 I took an intensive German language course at a place just outside Munich on a Goethe Institute Scholarship. One day it was raining hard and I got talking in a pub to a fellow who turned out to be an old Nazi. He said, 'You English should have gone into the war on our side. Together we would have destroyed Bolshevism and ruled the world.' And then he made a remark that chilled me—it still does. Der Jude ist eine Laus. 'The Jew is a louse.' That's when I first became intrigued by the Nazis."

Soon an established scholar of Nazi Germany, Kershaw was asked to write a biography of Hitler by Ravi Mirchandani, then an editor at Penguin UK. Initially, Kershaw demurred, deterred by the existence of the massive biographies by Alan Bullock and Joachim Fest. Rereading those books, however, he realized that neither focused on "what to me seemed the absolute pivotal thing, which was the relationship between race policy and this war of incredible destruction." The result was a massive two-volume life that is impossible to put down. Its authority lies partly in its masterful deployment of an encyclopedic range of sources, including many previously unavailable ones (such as the Goebbels diaries), and partly in the answer it provides to the conundrum of Hitler's leadership.

Earlier historians had argued for years over whether Hitler singlehandedly steered Germany onto its destructive Nazi course or merely reflected the ideological convictions of the German population at large. Kershaw feels neither view is adequate. He accepts that Hitler was a classic charismatic figure, capable of inspiring millions; equally, Hitler took little or no interest in the administration of anything other than military strategy (and that was to the despair of his generals). Yet Hitler galvanized his minions into a competitive struggle for his favor and into the murderous work that produced the Holocaust. His followers were, to use Kershaw's own formulation, "working towards the Fuhrer." And while never doubting the deep vein of anti-Semitism in German life, Kershaw argues that the apathy of many allowed the extremism of a fanatical few to prevail. In this sense, as Kershaw says, "the road to Auschwitz was paved with indifference."

A longtime professor at Sheffield University, Kershaw has lived for the past 30 years in the same house on the outskirts of Manchester, where his wife has worked in nursing education for many years. He's a family man, with grown sons and grandchildren who take Kershaw's fame as lightly as he does. "When Penguin brought out the last chapter of Nemesis [the second volume of Hitler] as one of the Penguin 70s series, Steven, my eldest, said, 'At last dad's written something I can read.' "

With his fame, offers have come from all over the world. Is he tempted ever by the larger salaries of American universities? "Not really," he says. "Various things have kept me in England that are more important to me than money—like rugby league and cricket. I'd love to visit for a term. I've always been excited by the intellectual engagement of people in the States."

So where does Sir Ian go from here—does he want to look further afield than the Second World War? "No," he says quietly. "I probably have one thing left to do that is important to me, a culminating work on the Nazi state that will be the opposite side of the coin of the Hitler biography. I want to show how the chaotic structure of the Nazi regime managed to function in such a lethal way. I suppose you could say it would be a kind of final hurrah."

He looks thoughtful as the waitress brings coffee, then asserts, "I still get a kick out of trying to communicate complex things to people who don't have the luxury of spending as much time as I do working on it."