The Age of Ideology that began in 1917 was an era in which the course of war was determined by the inevitabilist belief systems of the 19th-century counter-Enlightenment. A romantic, militarized nationalism inspired the expansionist programs of Fascist Italy and Imperial Japan, and with an additional dose of racialist pseudoscience, Nazi Germany. The leadership of each of these countries railed against the decadent individualism and universalism of the modern liberal West, and each was driven by the conviction that it was destined to rule over a natural domain: the Mediterranean, the Pacific rim, and the European continent, respectively. World War II began with invasions that were intended to move this destiny along. At the same time a romantic, militarized communism inspired the expansionist programs of the Soviet Union and China, who wanted to give a helping hand to the dialectical process by which the proletariat or peasantry would vanquish the bourgeoisie and establish a dictatorship in country after country. The Cold War was the product of the determination of the United States to contain this movement at something close to its boundaries at the end of World War II.

The brain trust of the 19th- and early-20th-century antiwar movement included Quakers such as John Bright, abolitionists such as William Lloyd Garrison, advocates of the theory of gentle commerce such as John Stuart Mill and Richard Cobden, pacifist writers such as Leo Tolstoy, Victor Hugo, Mark Twain, and George Bernard Shaw, the philosopher Bertrand Russell, industrialists such as Andrew Carnegie and Alfred Nobel (of Peace Prize fame), many feminists, and the occasional socialist (motto: "A bayonet is a weapon with a worker at each end"). Some of these moral entrepreneurs created new institutions that were designed to preempt or constrain war, such as a court of international arbitration in The Hague and a series of Geneva Conventions on the conduct of war.

But this narrative leaves out a major plot that perhaps had the most lasting impact on the 20th century. Mueller, Howard, Payne, and other political historians remind us that the 19th century was host to yet another movement: a continuation of the Enlightenment critique of war. Unlike the strain of liberalism that developed a soft spot for nationalism, this one kept its eye on the individual human being as the entity whose interests are paramount. And it invoked the Kantian principles of democracy, commerce, universal citizenship, and international law as practical means of implementing peace.

Peace first became a popular sensation with the publication of two bestsellers. In 1889 the Austrian novelist Bertha von Suttner published a work of fiction called Die Waffen nieder! (Lay Down Your Arms!), a first-person account of the gruesomeness of war. And in 1909 the British journalist Norman Angell published a pamphlet called Europe's Optical Illusion, later expanded as The Great Illusion, which argued that war was economically futile. Plunder may have been profitable in primitive economies, when wealth lay in finite resources like gold or land or in the handiwork of self-sufficient craftsmen. But in a world in which wealth grows out of exchange, credit, and a division of labor, conquest cannot make a conqueror richer. Minerals don't just jump out of the ground, nor does grain harvest itself, so the conqueror would still have to pay the miners to mine and the farmers to farm. In fact, he would make himself poorer, since the conquest would cost money and lives and would damage the networks of trust and cooperation that allow everyone to enjoy gains in trade. Germany would have nothing to gain by conquering Canada any more than Manitoba would have something to gain by conquering Saskatchewan.

For all its literary popularity, the antiwar movement seemed too idealistic at the time to be taken seriously by the political mainstream. Suttner was called "a gentle perfume of absurdity," and her German Peace Society "a comical sewing bee composed of sentimental aunts of both sexes." Angell's friends told him to "avoid that stuff or you'll be classed with cranks and faddists, with devotees of Higher Thought who go about in sandals and long beards, and live on nuts." H. G. Wells wrote that Shaw was "an elderly adolescent still at play… All through the war we shall have this Shavian accompaniment going on, like an idiot child screaming in a hospital." And though Angell had never claimed that war was obsolete -- he argued only that it served no economic purpose, and was terrified that glory-drunk leaders would blunder into it anyway -- that was how he was interpreted. After World War I he became a laughingstock, and to this day he remains a symbol for naïve optimism about the impending end of war. While I was writing this book, more than one concerned colleague took me aside to educate me about Norman Angell.

But according to Mueller, Angell deserves the last laugh. World War I put an end not just to romantic militarism in the Western mainstream but to the idea that war was in any way desirable or inevitable. "The First World War," notes Luard, "transformed traditional attitudes toward war. For the first time there was an almost universal sense that the deliberate launching of a war could now no longer be justified." It was not just that Europe was reeling from the loss of lives and resources. As Mueller notes, there had been comparably destructive wars in European history before, and in many cases countries dusted themselves off and, as if having learned nothing, promptly jumped into a new one. Recall that the statistics of deadly quarrels show no signature of war-weariness. Mueller argues that the crucial difference this time was that an articulate antiwar movement had been lurking in the background and could now say "I told you so."

The change could be seen both in the political leadership and in the culture at large. When the destructiveness of the Great War became apparent, it was reframed as "the war to end all wars," and once it was over, world leaders tried to legislate the hope into reality by formally renouncing war and setting up a League of Nations to prevent it. However pathetic these measures may seem in hindsight, at the time they were a radical break from centuries in which war had been regarded as glorious, heroic, honorable, or in the famous words of the military theorist Karl von Clausewitz, "merely the continuation of policy by other means."

World War I has also been called the first "literary war." By the late 1920s, a genre of bitter reflections was making the tragedy and futility of the war common knowledge. Among the great works of the era are the poems and memoirs of Siegfried Sassoon, Robert Graves, and Wilfred Owen, the bestselling novel and popular film All Quiet on the Western Front, T. S. Eliot's poem "The Hollow Men," Hemingway's novel A Farewell to Arms, R. C. Sherriff's play Journey's End, King Vidor's film The Big Parade, and Jean Renoir's film Grand Illusion -- the title adapted from Angell's pamphlet. Like other humanizing works of art, these stories created an illusion of first-person immediacy, encouraging their audiences to empathize with the suffering of others. In an unforgettable scene from All Quiet on the Western Front, a young German soldier examines the body of a Frenchman he has just killed:

No doubt his wife still thinks of him; she does not know what happened. He looks as if he would have often written to her -- she will still be getting mail from him -- Tomorrow, in a week's time -- perhaps even a stray letter a month hence. She will read it, and in it he will be speaking to her…

I speak to him and say to him: "… Forgive me, comrade… Why do they never tell us that you are poor devils like us, that your mothers are just as anxious as ours, and that we have the same fear of death, and the same dying and the same agony?"…

Another soldier asks how wars get started and is told, "Mostly by one country badly offending another." The soldier replies, "A country? I don't follow. A mountain in Germany cannot offend a mountain in France. Or a river, or a wood, or a field of wheat."

"I will write to your wife," I say hastily to the dead man…"I will tell her everything I have told you, she shall not suffer, I will help her, and your parents too, and your child --" Irresolutely I take the wallet in my hand. It slips out of my hand and falls open… There are portraits of a woman and a little girl, small amateur photographs taken against an ivy-clad wall. Along with them are letters.

The upshot of this literature, Mueller notes, was that war was no longer seen as glorious, heroic, holy, thrilling, manly, or cleansing. It was now immoral, repulsive, uncivilized, futile, stupid, wasteful, and cruel. And perhaps just as important, absurd. The immediate cause of World War I had been a showdown over honor. The leaders of Austria-Hungary had issued a humiliating ultimatum to Serbia demanding that it apologize for the assassination of the archduke and crack down on domestic nationalist movements to their satisfaction. Russia took offense on behalf of its fellow Slavs, Germany took offense at Russia's offense on behalf of its fellow German speakers, and as Britain and France joined in, a contest of face, humiliation, shame, stature, and credibility escalated out of control. A fear of being "reduced to a second-rate power" sent them hurtling toward each other in a dreadful game of chicken.

I'd be unworthy of the high trust that's been placed in me if I didn't do everything within my power to keep our beloved Freedonia at peace with the world. I'd be only too happy to meet Ambassador Trentino and offer him on behalf of my country the right hand of good fellowship. And I feel sure he will accept this gesture in the spirit in which it is offered.

Contests of honor, of course, had been setting off wars in Europe throughout its bloody history. But honor, as Falstaff noted, is just a word -- a social construction, we might say today -- and "detraction will not suffer it." Detraction there soon was. Perhaps the best antiwar film of all time is the Marx Brothers' Duck Soup (1933). Groucho plays Rufus T. Firefly, the newly appointed leader of Freedonia, and is asked to make peace with the ambassador of neighboring Sylvania:

But suppose he doesn't. A fine thing that'll be. I hold out my hand and he refuses to accept it. That'll add a lot to my prestige, won't it? Me, the head of a country, snubbed by a foreign ambassador. Who does he think he is that he can come here and make a sap out of me in front of all my people? Think of it. I hold out my hand. And that hyena refuses to accept it. Why, the cheap, four-flushing swine! He'll never get away with it, I tell you! [The ambassador enters.] So, you refuse to shake hands with me, eh? [He slaps the ambassador.]

Ambassador: Mrs. Teasdale, this is the last straw! There's no turning back now! This means war!

The butt of the joke was different in the other classic war satire of the era, Charlie Chaplin's The Great Dictator (1940). It was no longer the hotheaded leaders of generic Ruritanian countries that were the target, since by now virtually everyone was allergic to a military culture of honor. Instead the buffoons were thinly disguised contemporary dictators who anachronistically embraced that ideal. In one memorable scene, the Hitler and Mussolini characters confer in a barbershop and each tries to dominate the other by raising his chair until both are bumping their heads against the ceiling.

Whereupon an outlandish production number breaks out in which the Marx Brothers play xylophone on the pickelhauben of the assembled soldiers and then dodge bullets and bombs while their uniforms keep changing, from Civil War soldier to Boy Scout to British palace guard to frontiersman with coonskin cap. War has been likened to dueling, and recall that dueling was eventually laughed into extinction. War was now undergoing a similar deflation, perhaps fulfilling Oscar Wilde's prophecy that "as long as war is regarded as wicked, it will always have its fascination. When it is looked upon as vulgar, it will cease to be popular."

By the 1930s, according to Mueller, Europe's war aversion was prevalent even among the German populace and its military leadership. Though resentment of the terms of the Treaty of Versailles was high, few were willing to start a war of conquest to rectify them. Mueller ran through the set of German leaders who had any chance of becoming chancellor and argued that no one but Hitler showed any desire to subjugate Europe. Even a coup by the German military, according to the historian Henry Turner, would not have led to World War II. Hitler exploited the world's war-weariness, repeatedly professing his love of peace and knowing that no one was willing to stop him while he was still stoppable. Mueller reviews biographies of Hitler to defend the idea, also held by many historians, that one man was mostly responsible for the world's greatest cataclysm:

After seizing control of the country in 1933, [Hitler] moved quickly and decisively to persuade, browbeat, dominate, outmaneuver, downgrade, and in many instances, murder opponents or would-be opponents. He possessed enormous energy and stamina, exceptional persuasive powers, an excellent memory, strong powers of concentration, an overwhelming craving for power, a fanatical belief in his mission, a monumental self-confidence, a unique daring, a spectacular facility for lying, a mesmerizing oratory style, and an ability to be utterly ruthless to anyone who got in his way or attempted to divert him from his intended course of action…

Fifty-five million deaths later (including at least 12 million who died in Japan's own atavistic campaign to dominate East Asia), the world was once again in a position to give peace a chance.

Hitler needed the chaos and discontent to work with -- although he created much of it, too. And surely he needed assistance -- colleagues who were worshipfully subservient; a superb army that could be manipulated and whipped into action; a population capable of being mesmerized and led to slaughter; foreign opponents who were confused, disorganized, gullible, myopic, and faint-hearted; neighbors who would rather be prey than fight -- although he created much of this as well. Hitler took the conditions of the world as he found them and then shaped and manipulated them to his own ends.