President John F. Kennedy once remarked that “in 1914, with most of the world already plunged in war, Prince Bulow, the former German chancellor, said to the then-chancellor Bethmann-Hollweg: ‘How did it all happen?’ And Bethmann-Hollweg replied: ‘Ah, if only one knew.’ If this planet is ever ravaged by nuclear war,” Kennedy went on, “if the survivors of that devastation can then endure the fire, poison, chaos and catastrophe, I do not want one of those survivors to ask another, ‘How did it all happen?’ and to receive the incredible reply, ‘Ah, if only one knew.’ ”

The anecdote about World War I came from Barbara Tuchman’s best-selling history “The Guns of August,” in which Tuchman explored the immediate origins and first weeks of the war. The book inspired Kennedy to install a tape system in the White House, including the Oval Office, to ensure an accurate record of decision-making. It was still on his mind as he confronted the Cuban missile crisis. “I am not,” the president told his brother Bobby, “going to follow a course which will allow anyone to write a comparable book about this time: ‘The Missiles of October.’ ”

Where Tuchman influenced President Kennedy and the popular imagination, Fritz Fischer, a year earlier, had become the touchstone for historians. His hugely controversial account, “Germany’s Aims in the First World War,” published in English in 1967, accused Germany of intentionally starting the war. Historians since have all weighed in on the blame game. Of recent scholarship, Max Hastings backs Fischer in holding Germany responsible; Sean McMeekin argues it was Russia’s fault; Niall Ferguson points the finger at Britain; while Christopher Clark shows Europe “sleepwalking” into war. Despite these bold and often compelling accounts, the case remains unsettled.

The scale of the disaster that followed the events of August 1914 complicates the historian’s task. “Loss of a generation” was a lament heard around Europe when the war was over. The conflict claimed 20 million military and civilian lives, with a further 21 million wounded. For some countries the burden was greater than others. While Britain, France and Germany lost between 2 and 3 percent of their total populations, Serbia suffered a staggering 15 percent depletion. Such losses had seemed unthinkable when the war began.

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