Today is the 100th anniversary of the official start of World War I.

Peggy Noonan:

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.

As Miranda Carter shows in George, Nicholas and Wilhelm (2010), the king of England, the czar of Russia and the kaiser of Germany—all cousins, all grandchildren of Victoria—were all in different ways wrecked by the war.

Kaiser Wilhelm, whose bombast, peculiarities of personality and lack of wisdom did so much to bring the conflict, folded almost from the start. Two years in, he was described by those around him as a “broken man”—depressed, lethargic, ill. An aide wrote of him as “violent and unpredictable.”

Barbara Tuchman, in the classic The Guns of August (1962), notes how in the early days of the war Wilhelm’s margin notes on telegrams became “more agitated.” (“Rot!” “He lies!” “False dog!”) In time, top brass shunted him aside and viewed him as irrelevant. The kaiser rarely referred to the sufferings of his people. Ms. Carter writes: “Wilhelm had always had difficulty in empathizing with others’ difficulties.” When his country collapsed, he fled to Holland, where in conversation he referred to his countrymen as “pigs” and insisted that the war was the fault of others. He died at age 81 in 1941, two years into the World War II.

King George V did have empathy, and it almost killed him. Touring the Western Front, he suffered at the sights—once-rich fields now charred craters, villages blasted away, piles of dead bodies. He aged overnight, his beard turning almost white. Ms. Carter writes that he now surveyed the world with a “dogged, melancholic, unsmiling stare.” A year into the war, a horse he was riding on a visit to the front got frightened, reared, and fell on him. The king never fully recovered from the injuries. Years later, he was haunted by what he called “that horrible and unnecessary war.” In 1935, war clouds gathering once again, he met up with his wartime prime minister. The king, wrote Lloyd George, “broke out vehemently, ‘And I will not have another war, I will not.’ ” He also said that the Great War had not been his fault. He died the following year.

Czar Nicholas II of Russia, of course, would lose everything—his throne and his life, as his family would lose theirs. But from the early days of the war he too was buckling. His former chief minister, Vladimir Kokovtsov, called Nicholas’s faded eyes “lifeless.” In the middle of conversations, the czar lost the thread, and a simple question would reduce him to “a perfectly incomprehensible state of helplessness.”

Two years in, Kokovtsov thought Nicholas on the verge of nervous breakdown. So did the French ambassador, who wrote in the summer of 1916: “Despondency, apathy and resignation can be seen in his actions, appearance, attitudes and all the manifestations of the inner man.” The czar wore a constant, vacant smile, but glanced about nervously. Friendly warnings that the war was not being won and revolution could follow were ignored. For him, in Ms. Carter’s words, “Contradiction now constituted betrayal.” At the end, those close to Nicholas wondered if he failed to move to save his throne because he preferred a crisis that might force his abdication—and the lifting of burdens he now crushingly understood he could not sustain.

Then there is Woodrow Wilson at his second Versailles peace conference, in the spring of 1919. Negotiations were draining, occasionally volatile. The victors postured, schemed and turned on each other for gain. They had literally argued about whether windows should be opened, and about what language should be the official one of the talks. (They settled on three.) President Wilson developed insomnia and a twitch on the left side of his face. He was constantly tired, occasionally paranoid. After a trying meeting with France’s finance minister, Louis Klotz, Wilson joked with a friend of his weariness: “I have Klotz on the brain.” …

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.

World War I was claimed to be the “war to end all wars.” World War II, 25 years later, disproved that. Historians also are coming to the view that 1919 through the mid-1930s was more a pause than peace. Certainly the peace settlement for the first war helped pave the way for the second war, though the rise (through democratic means, by the way) of Adolf Hitler had more to do with World War II.