By Peggy Noonan

Next week marks the 100th anniversary of the start of World War I. It was the great disaster of the 20th century, the one that summoned or forced the disasters that would follow, from Lenin and Hitler to World War II and the Cold War. It is still, a century later, almost impossible to believe that one event, even a war, could cause such destruction, such an ending of worlds.

History still isn’t sure and can never be certain of the exact number of casualties. Christopher Clark, in “The Sleepwalkers” (2013), puts it at 20 million military and civilian deaths and 21 million wounded. The war unleashed Bolshevism, which brought communism, which in time would kill tens of millions more throughout the world. (In 1997, “The Black Book of Communism,” written by European academics, put the total number at a staggering 94 million.)

Thrones were toppled, empires undone. Western Europe lost a generation of its most educated and patriotic, its future leaders from all classes—aristocrats and tradesmen, teachers, carpenters and poets. No nation can lose a generation of such men without effect. Their loss left Europe, among other things, dumber.

Reading World War I histories, I have been startled to realize the extent to which the leaders or putative leaders of the belligerent nations personally suffered. A number of them fell apart, staggering under the pressure, as if at some point in the day-to-day they realized the true size and implications of the endeavor in which they were immersed. They seemed to come to understand, after the early hurrahs, that they were involved in the central catastrophe of the 20th century, and it was too big, too consequential, too history-making to be borne. Some would spend the years after the war insisting, sometimes at odd moments, that it wasn’t their fault.

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So what are we saying? Nothing beyond what I suppose has long been a theme, which may be a nice word for preoccupation, in this space: History is human.

And sometimes it turns bigger than humans can bear.