The orthodox chronology of the second World War that classifies it as a global conflict beginning with Germany’s invasion of Poland in 1939 and ending with Japan’s surrender in 1945 has fallen out of favour among historians. So, too, has the idea that the war’s primary cause was Adolf Hitler’s grab for world power.

Nowadays the preferred concept is that of a “long” second World War, with some historians dating its outbreak to Japan’s invasion of Manchuria in 1931 or its attack on China in 1937. Others frame the conflict as merely one phase in a 30-year-long European civil war that began in 1914. Equally, both world wars were followed by chaotic peace in which millions died during the course of ferocious inter-ethnic and political struggles – conflicts that lingered for decades.

Prof Richard Overy is the author of several classic studies of the second World War, notably Why The Allies Won (1995). In that book he challenged the idea that Hitler’s defeat was preordained because of the preponderance of Allied military, economic and industrial power. The Allies won, he argued, because they turned their objective strengths into effective fighting power. Vital to that success was the mobilisation of a morally-charged popular will to win. All sides to the conflict considered their cause just – even the Nazis – but Hitler’s Axis alliance could not match the moral certainty of the Allies, who won because they fervently believed they had right as well as might on their side.

The victors’ justice meted out at the Nuremberg trials of major Nazi war criminals was founded on the reality that Hitler planned and waged aggressive wars that had catastrophic consequences.

Underlying these seemingly simple truths are many layers of complexity, not least the Soviet Union’s decisive role in defeating Nazi Germany. After the war, half of Europe came under Joseph Stalin’s authoritarian sway. Better Stalin’s communist empire than Hitler’s racist-genocidal regime, but the war’s outcome was not a triumph for liberal democracy.

In his latest book, Overy reprises, updates and expands his coverage of the war. His masterly synthesis of the war’s vast literature and sources has never been bettered. The text may be long but it is unflagging and consistently illuminating. Overy’s narrative is enlivened by personal accounts of the wartime experience and the book’s many statistics tell their own story.

More than 120 million men and women served in armed forces during the war. The Soviets and Americans produced 443,000 planes, 175,000 tanks and 676,000 artillery pieces. About 18 million German, Japanese and Soviet soldiers died, compared to a million from western states. By February 1942, two million Soviet POWs had died in German captivity. A total of 900,000 civilians were killed by American, British and Canadian bombing raids, including 140,000 citizens of liberated states. In the Pacific theatre, only 41,000 Japanese soldiers and sailors were taken alive. In France, 60 per cent of Jews detained for deportation to Auschwitz and other murder camps were arrested by French police. Hundreds of thousands of women were raped by members of conquering Allied armies, most of them by the Soviets. Around 40 per cent of American veterans received disability compensation for psychiatric disorders. After the war, millions of Germans, Japanese and Italians were ethnically cleansed from lands their ancestors had lived in for generations.

Two-thirds of the 50-60 million people who died were civilians but it was their active involvement that made the war a total one: “A remarkable feature of the wartime years”, writes Overy, “was the willingness of civilians to act on their own behalf against air attack, or invasion and occupation, or, in the extreme case of the Jewish genocide, against the menace of extermination”.

He characterises the conflict as “the last imperial war” impelled by the German, Italian and Japanese drive for territorial empires to match those of Britain and France: “The other factors usually emphasised in analysing the origins of the second World War – the arms race, diplomatic crises, ideological conflict – were effects of the new wave of empire-building, not causes”, argues Overy. It was the British and French declaration of war on Germany that turned Hitler’s local war in Poland into a major European war: “The second World War was the result of decisions taken in London and Paris, not Berlin.”

For British and French leaders, going to war was a necessary re-fighting of the first World War to secure a more resilient international order guaranteeing a permanent peace not only in Europe but throughout their global empires. The rapid fall of France in 1940 upset that calculation and for a time it seemed the Axis empires would become new rulers of the world.

Overy’s narrative of the long second World War begins with the rise of “nation-empires” at the end of the 19th century and concludes with the violent postwar dissolution of the British, French, Belgian and Dutch empires. The first World War was, says Overy, an imperial war whose result the Axis states strove to reverse from the early 1930s onwards. They failed, but only because they were defeated by two anti-imperial great powers – the Soviet Union and the US. The most fundamental result of that Allied victory was the definitive end of the imperial project and its replacement by a world based on nation states.

Since no war’s consequences were more devastating or far-reaching than the second World War the idea the conflict must have had equally profound causes is intuitively appealing. But Overy provides abundant evidence that both world wars were the result of chance, contingency, accident and personality.

Above all, there was the Hitler factor. It was his miscalculations that led to the outbreak of a major European war. It was his unexpected victories in Europe that undermined western imperial power and encouraged Japanese expansionism in Asia. It was his striving for a worldwide Axis alliance with Italy and Japan, together with his attack on Russia and his declaration of war on the US, that crystallised the global war. It was the Nazis’ barbarous campaign on the eastern front that made the war so atrocious. It was Hitler’s virulent anti-Semitism that begat the Holocaust.

Without this human causal chain we might have been spared the blood and ruin caused by the “Great Imperial War” but be living still in a world dominated by imperialism and empires.

Geoffrey Roberts is Emeritus Professor of History at UCC and a Member of the Royal Irish Academy. His Stalin’s Library: A Dictator and His Books will be published by Yale University Press in February 2022