The world in which we still live was forged in the furnace of World War II — yet that war now seems to belong to another world. No war is more familiar to us, but its moral assumptions appear as remote as those of Homer. It was, by common consent, both a “just” war — the last to be generally recognized as such — and one in which heroism was not yet seen as an anachronism. Its origins, course, and consequences were determined, not by the impersonal forces beloved of academic discourse, but primarily by a handful of individuals.

Not one of the 10 “Fateful Choices,” the title of Ian Kershaw’s new book (Penguin Press, 656 pages $35) was primarily a military decision. It would be possible to write another book about those who were actually going up in smoke rather than about the statesmen in their smoke-filled rooms. (Adolf Hitler was the only non-smoker among them.) But this is a work of political rather than military history by a bi ographer of Hitler. His purpose is to ask: What were the options open to these leaders, and why did they make the choices they did?

Mr. Kershaw’s fateful choices were all made during the period between May 1940 and December 1941. One may legitimately question this time-frame: It excludes equally momentous decisions before and after, such as that of Hitler to invade Poland, or that of President Truman to drop nuclear bombs on Japan.

One may even quibble with the selection within his chosen period. A case could be made, for example, that Francisco Franco’s decision to stay out of the war in 1940 was just as important to its outcome as Benito Mussolini’s decision to enter it. Mr. Kershaw dismisses this contention, but the late Hugh Trevor-Roper did indeed make a persuasive case that Franco, by denying the British access to the Mediterranean, could have won the war for the Axis.

It is also doubtful whether Mr. Kershaw has really identified 10 separate decisions. He gives a chapter to each one, but there is some double counting. He devotes two chapters to President Franklin Roosevelt’s decision to support the British against Germany. Yet Roosevelt did not in the end need to make the decision to go to war: Japan and Germany saved him the trouble. Japan also rates two chapters for what was essentially one decision: to go to war against America and the British Empire.

As for Joseph Stalin: It is not clear why his decision to ignore warnings of an imminent German attack — the one chosen by Mr. Kershaw — was more important than, say, his decision to sign a pact with Germany in August 1939, which allowed Hitler to go to war, or his decision to remain in Moscow during the German assault in October 1941, which Vyacheslav Molotov, among others, believed saved the capital and the war.

Mr. Kershaw singles out three of Hitler’s decisions: to invade the Soviet Union, to declare war on America, and to kill the Jews of Europe. It is hard to argue with the significance any of these, and yet Hitler did make other decisions during 1940–41 that were nearly as pivotal. Having failed to knock the British out of the war, he chose not to offer them generous peace terms — though Stalin was not the only one to suspect that Rudolf Hess had been sent to do just that in May 1941.

Would the British then have been interested in a deal with Hitler? Mr. Kershaw thinks they might have been a year earlier. For a crucial three days at the end of May 1940, the new prime minister, Winston Churchill, wrestled with the appeasers in his cabinet, as the fall of France and the loss of the British expeditionary force holed up in Dunkirk seemed imminent.

The foreign secretary, Lord Halifax, wanted to make overtures to Mussolini, who had not yet declared war and might have acted as a mediator. Mr. Kershaw quotes Halifax’s version of Churchill’s surprising response: “If Herr Hitler was prepared to make peace on the terms of the restoration of the German colonies [confiscated after World War I] and the overlordship of Central Europe, that was one thing. But it was quite unlikely that he would make any such offer.”

Of course, Churchill had no intention of accepting such terms. His pragmatism was purely tactical: Churchill needed to persuade Neville Chamberlain, still a key figure in his party and his war cabinet, to support the no surrender policy.

In any case, Churchill was right: Hitler was intoxicated with victory. A British approach via Mussolini would have been interpreted by him as tantamount to capitulation. But Mr. Kershaw is mistaken to suggest that public opinion played no part in Churchill’s decision to fight on. However desperate their plight, the British were not ready to accept defeat, as the French had done. Lloyd George hoped to play the role of a British Pétain, but the call never came.

Even before the “miracle” of Dunkirk and his immortal oratory during the Battle of Britain, Churchill had the nation with him.

Despite these caveats, this is a thoughtful and entertaining book: an excellent synthesis of recent scholarship which wears its learning lightly. Mr. Kershaw impressively traces the global repercussions that each decision had on the others. Mussolini’s disastrous attack, first on Albania and then on Greece, set off a chain reaction that led Hitler to postpone his Russian campaign. Japan’s Pacific war resolved Roosevelt’s dilemma over how to turn America’s undeclared Atlantic war into a declared one.

Most baffling of all was Hitler’s declaration of war on America. Mr. Kershaw shows the genesis of this catastrophic decision, arguing that it had become inevitable. Hitler received nothing in return from the Japanese, who refused to attack the Russians in Siberia and did not even warn him about Pearl Harbor. It was his own persistent underestimation of the Americans that proved fatal. Some of his less toadying officials were appalled: They had, said one general, “never even considered a war against the United States.”

Mr. Kershaw draws no conclusions from this story, but readers may like to consider whether other, more recent enemies of democracy share Hitler’s tendency to underestimate the English-speaking peoples. It is often assumed that dictatorships are more decisive than democracies. Yet it is striking that, of the big decisions examined in this book, those made by Hitler, Stalin, Mussolini, and Hideki Tojo were all ultimately suicidal, whereas the decisions of Churchill and Roosevelt were farsighted and wise. Democratic leaders often lose battles against dictators. They rarely lose the war.

Mr. Johnson last wrote for these pages on the historian Andrew Roberts.