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It was the year that marked the turning point of the Second World War and shaped the post-war world. 1941 began with Germany victorious in Western Europe and challenged only by a Britain that was bereft, after the fall of France, of a continental ally. Churchill remained defiant, but any hope of victory seemed dependent on the scenario of Britain finding a new "continental sword" in the shape of the Soviet Union, and an entry into the war by the US. Neither seemed likely, as the pact between Germany and the USSR appeared firm and Franklin D. Roosevelt had just won a presidential election on a platform of keeping the US out of the war.

But two months, June and December, transformed the strategic situation. On 22 June, Germany launched Operation Barbarossa, the invasion of the Soviet Union, and on 7 December aircraft from Japanese carriers attacked the US naval base of Pearl Harbor. It is the first 12 days of the 12th month that are the subject of Evan Mawdsley's study.

The die had already been cast when the Imperial Conference took place at the Imperial Palace in Tokyo on 1 December, for Japanese troops and ships had, for several weeks, been moving into the positions from which they would strike. The statesmen, generals and admirals of Japan met to ratify rather than make the decision to begin war against the US, the British Empire and the headless colonies of a Netherlands occupied by Germany. They did so in what they saw as the interests of the Japanese Empire, whose very survival they felt was imperilled, and in the name of their emperor, whom Mawdsley describes as sitting "silent throughout, on a raised seat before a gold screen", although he nodded in agreement to each explanation for the momentous decision.

The events of the next 11 days are described in exhaustive detail in what constitutes a brilliant exercise in military history. After trawling through many archives and consulting numerous monographs and articles, Mawdsley has managed not only to assemble the facts about each fighting unit, both Allied and Japanese, their chains of command and their strengths, but to utilise them for an account of naval and army operations that is fresh and vivid. He maintains a close focus on the events that led to the new theatre of the war and on Japan's military operations, while not losing sight of developments in the war in Europe. Indeed, a strength of this book is the discussion of events on Germany's eastern front - where Barbarossa was already faltering in the face of the unexpected capacity for recovery displayed by Soviet forces - in parallel with Japan's operations in the Pacific.

The overwhelming success of Japanese forces at the beginning of the war in the Far East in their attacks on Hawaii, the Philippines, Hong Kong, the Dutch East Indies, Malaya and Burma is a well-known story, but Mawdsley's account is authoritative, invigorated by new perspectives and contextualised in great themes: the end of empires, the dawn of American might and the struggle for supremacy in Southeast Asia.

Important questions as to motivations and war aims hang over any account of Japan's entry into the Second World War. Alternative options were open to Japan, Germany and the US. Instead of going south, Japan could have gone north, a course long favoured by the army, and attacked the Soviet Union from Manchuria at a time when German troops were deep in Soviet territory, but Hitler gave the Japanese no encouragement to do so. Roosevelt's actions, particularly the economic sanctions he placed on Japan, raise the question of whether he manoeuvred Japan into attacking the US as part of a strategy that would enable him to bring a reluctant America into the European war. And why, having obliged Roosevelt by declaring war on the US, did Germany never seek to coordinate its war with Japan's? Mawdsley provides fascinating information on Roosevelt's policies and decisions, but devotes far less space to the lack of coordination between the Axis partners than this intriguing subject surely warrants.

What is undeniable is that the events of early December 1941, which this book describes so well, brought the world's most powerful economy and a nation with enormous military potential into the war on Britain's side. Britain was soon to experience humiliating defeats in the Far East, but on the evening of 7 December, his hopes of new allies having been fulfilled, Churchill, on going to bed, "slept the sleep of the saved and thankful".