This essay is adapted from the Rede Lecture, which was delivered, earlier this year, at Cambridge University.

A hundred years ago this past August, Europe exploded in a conflagration that came to engulf the world in a new kind of war. The Great War, it was called, until a subsequent and even more devastating conflict made it seem perhaps just the first in a series, part of a new genre of human destruction and cruelty.

Those who have written about the Great War in the hundred years since it began, and those so deeply engaged in commemorating it today, have frequently seen it as a—even _the—_defining event of the twentieth century, and of modernity more broadly. George Kennan described the war as “the great seminal catastrophe” of the century, one that sowed the seeds not just of the Second World War but of Communism, Nazism, the Holocaust, and the Gulag. It was, in the words of other analysts and scholars, “a historical watershed,” “iconic, a symbol of the catastrophic character of the twentieth century as a whole.” It brought “a novel type of warfare”; “the Western World’s entire relationship to war was permanently and drastically altered.” Wyndham Lewis, the British writer, painter, and artillery officer, proclaimed the war “the turning point in the history of the earth.” As one prominent historian put it, just this year, it was “the greatest black swan event in world history”—an event both unimaginable and transformative. The Great War introduced a level of brutality and revealed a capacity for human cruelty that has shaped our lives ever since. Particularly in Europe, the First World War is seen to have introduced a rupture into the history of war and the history of human experience, a rupture that is fundamental to who we are today, a full century after those fateful August guns began to sound.

But as we think about the Great War and its meaning, and as we consider the history both of warfare and of modern society, we should remember another war that we are also in the midst of commemorating. A hundred and fifty years ago this past September, fifty years before the Great War began, William Tecumseh Sherman took Atlanta, a much coveted Union objective. It was the fourth year of a war whose devastation no one had anticipated and which no one could have imagined. Sherman’s victory left him poised for what became his infamous March to the Sea, initiated just weeks later. His determination to make civilians—“old and young, rich and poor”—feel “the hard hand of war” has made his march notorious, and indeed revolutionary, in the annals of warfare, a symbol of the new levels of destructiveness made possible by changed technologies, economies, and societies.

A case can be made that the American Civil War anticipated, in important ways, the transformations that have so often been attributed to the years between 1914 and 1918. The Civil War might well be viewed as the beginning of a “long twentieth century”: in its introduction of a scale of death that came to be associated with a later era, in what Williamson Murray and MacGregor Knox, historians of the First World War, have called warfare’s new combination of “industrial firepower and logistics with the fighting power and staying power that nationalism could generate”; in its mobilization of mass armies through the novel introduction of conscription; in its associated reliance on citizens both in battle and on the home front to sustain the conflict; and in a resulting emergence of new conceptions of citizenship and its privileges, which affected both the living and the dead. The violence of the American Civil War interrupted an age that saw itself as one of growing benevolence and humanitarianism, introducing a startling awareness of man’s capacities for destruction that found its terrible fulfillment between 1914 and 1918.

When I began exploring this comparison, I was struck by the similarity of two numbers. The best calculation of the number of men from the British Isles who died in the Great War is 722,785. Although records from the Civil War are inaccurate and incomplete, the most recent and sophisticated analyses indicate that approximately seven hundred and fifty thousand soldiers, from the North and the South, perished between 1861 and 1865. The size of the British population in 1914 was somewhat larger than that of the United States in 1860, so the percentage of loss in the two countries differed: about 1.6 per cent in Britain and about 2.5 per cent in the United States. But to think of these two wars in terms of the similar numbers of foreshortened lives, and of the resulting circles of mourners and bereaved, was revelatory. So let us take these numbers as a starting point, as a shared foundation of loss, and as a way into exploring two wars through the lens of two nations, two societies that have come to regard them as defining moments.

More Americans died in the Civil War than in all other American wars combined, from the Revolution through Vietnam. The death toll was both stunning and unanticipated by combatants who, in both the North and the South, had at the outset expected the conflict to be of short duration and little human cost. In Britain, the losses of the Great War were similarly staggering and relentless, averaging four hundred and fifty-seven dead a day through the war, and reaching an apogee on the first day of the Battle of the Somme, in July, 1916, still the bloodiest day in the history of the British Army, with more than nineteen thousand dead and nearly forty thousand wounded or missing. This scale of death, and the pervasive grief it inflicted, resulted from significant innovations in warfare. So many people died partly because so many were mobilized to fight. These were mass armies—as many as two million Americans in arms in the eighteen-sixties and 5.7 million British in the Great War—that were made possible by both ideology and technology. In both conflicts, nationalism spurred the participation of hundreds of thousands of civilian soldiers, who created armies quite unlike any that had preceded them. The American North was fighting to preserve a nation, the Confederacy to establish one, Britain to defend one. These conflicts were cast and widely perceived as people’s wars, even though that faith began to erode across Europe in the latter years of the First World War. But it was nevertheless the infusion of ideology and nationalist commitment into warfare that marked one of the most significant dimensions of war’s new character.

In both wars, however, enthusiasm proved insufficient to sustain the necessary manpower supply, and both Britain and America would, for the first time, introduce conscription. In the Civil War, the Confederacy adopted an initial conscription bill in April, 1862, and the Union followed eleven months later. Britain was a year and a half into the Great War when it passed the Military Service Bill of 1916. About fifty-seven per cent of those who served were drafted, a far higher percentage than had been the case in the United States half a century earlier. Over all, about a quarter of the adult male population in Britain entered the military, compared with three-quarters of white men of military age in the South and forty per cent in the Union. But, in both Britain and America, the many millions of men who joined the military created the conditions for a new kind of warfare, as well as the necessity for a new understanding of citizenship and its privileges in the aftermath of such widespread sacrifice. In the United States, for example, the wartime courage and service of African-American soldiers played a critical part in the movement to expand the franchise and pass the Fifteenth Amendment, which prohibited denial of the right to vote “on account of race, color, or previous condition of servitude.” In Britain, the slogan “One Gun, One Vote” captured the public sentiment that led to the Representation of the People Act in 1918, tripling the size of the British electorate. Similar expansions of the franchise were enacted across Europe after the Great War.