The Second World War did not begin in 1939. A. J. P. Taylor was wont, with his mixture of impishness and insight, to argue that all the fighting in Europe until the German attack on the Soviet Union on June 22, 1941, could be seen as a succession of independent wars.

Historians of the Far East have similar difficulties with the textbook focus on 1939. Japan exploited the incident on the Marco Polo bridge to engineer war with China in 1937, and Ian Kershaw in Fateful Choices traces the war's origins to the Mukden incident in 1931.

But the Second World War did not begin in 1931 or 1937, any more than it did in 1939. It began in 1941. When Germany received the news of the Japanese attack on Pearl Harbor on December 8, Hitler and his circle were exultant. "This has become a world war in the truest meaning of the word," Goebbels wrote in his diary.

Kershaw's book, despite its pot-boiling title and its fondness for adjectives such as "fatal" and "inevitable", is a scholarly, important and frequently revisionist account of the origins of the "real" Second World War, the stages by which two regional wars became united in one greater conflict. Not the least of his achievements is to make clear the artificiality of any division between Asia and Europe.

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Each of Kershaw's ten chapters follows a single decision from the perspective of a single power. But for each of those powers no decision could stand in isolation.

Hitler welcomed the opportunity to declare war on the US, which the Japanese attack on Pearl Harbor afforded, because of the situation in the Atlantic and the Soviet Union. The "undeclared war" between Germany and the US prevented the full implementation of the U-boat campaign, and the stalled offensive in Russia made it improbable that Germany would complete its conquest of Europe before the US entered the war on Britain's side.

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Thus Hitler's seemingly illogical rush to declare war on America was entirely rational. He reckoned that the US would be sucked into the Pacific, and the armaments that it was providing to Britain and the Soviet Union would be diverted to America's own build-up.

In other words, single states employed global strategies and took decisions in the light of them. Churchill persuaded his cabinet colleagues that Britain should fight on because he believed that resistance would secure the support of the US. Hitler decided to invade the Soviet Union in part because he attributed Britain's stubbornness to its hope that Russia would fight Germany, and because he anticipated that by doing so he would free Japan to tie down the US.

Both Britain and Germany knew what they were letting themselves in for. Goebbels's readiness to describe what followed as a "total war" was mirrored by Churchill's description of what students of strategy now call existential war. "Nations which went down fighting", he told the war cabinet on May 28 1940, "rose again, but those which surrendered tamely were finished".

From the lips of a democratically elected leader fighting against Nazi tyranny such words seem legitimate. But when articulated by Hitler for Germany or Tojo for Japan they are evidence of the pursuit of war for its own sake, almost devoid of political objectives.

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Ironically, the least rational of all the decisions described in such detail by Kershaw (with the significant but somewhat different decision by Germany to kill the Jews) was the only truly collective decision for war by any of the belligerents, that of Japan. Here the armed forces, noticeable elsewhere for their lack of influence, had a disproportionate voice. Spurred on by middle ranking staff officers working on best-case scenarios, but divided by the rivalry between the army and the navy, they exploited their right of direct access to the emperor to put a case for war with the US, although they knew that they had little chance of winning it.

Kershaw stresses his rejection of counter-factual approaches to history, although in Japan's case in particular he finds it hard to resist the temptation. His is an account where public opinion - with the exception of Roosevelt's need to take account of isolationism in the US - plays little part, and where each step is largely shaped by the one that has gone before it. But he is too good a historian to discount the role of contingency.

Ultimately Churchill needed the "miracle of Dunkirk" to validate his premiership as well as his policy, just as Roosevelt needed Pearl Harbor to be sure of the endorsement of Congress. Here is the history of war written from the perspective of political elites, sure in their own judgements, if less certain of the outcomes that would ensue. Until December 1941 nobody could feel confident of ultimate victory.

Hew Strachan is professor of the history of war, Oxford University, and director of the Leverhulme programme on the changing character of war.

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