Oxford, England — Earlier this year, I was on holiday in Corsica and wandered into the church of a tiny hamlet in the hills where I found a memorial to the dead from World War I. Out of a population that can have been no more than 150, eight young men, bearing among them only three last names, had died in that conflict. Such lists can be found all over Europe, in great cities and in small villages. Similar memorials are spread around the globe, for the Great War, as it was known before 1940, also drew soldiers from Asia, Africa and North America.

World War I still haunts us, partly because of the sheer scale of the carnage — 10 million combatants killed and many more wounded. Countless civilians lost their lives, too, whether through military action, starvation or disease. Whole empires were destroyed and societies brutalized.

But there’s another reason the war continues to haunt us: we still cannot agree on why it happened. Was it caused by the overweening ambitions of some of the men in power at the time? Kaiser Wilhelm II and his ministers, for example, wanted a greater Germany with a global reach, so they challenged the naval supremacy of Britain. Or does the explanation lie in competing ideologies? National rivalries? Or in the sheer and seemingly unstoppable momentum of militarism? As an arms race accelerated, generals and admirals made plans that became ever more aggressive as well as rigid. Did that make an explosion inevitable?...

The search for explanations began almost as soon as the guns opened fire in the summer of 1914 and has never stopped. The approaching centenary should make us reflect anew on our vulnerability to human error, sudden catastrophes, and sheer accident. History, in the saying attributed to Mark Twain, never repeats itself but it rhymes. We have good reason to glance over our shoulders even as we look ahead. If we cannot determine how one of the most momentous conflicts in history happened, how can we hope to avoid another such catastrophe in the future?...