At the same time, there have always been eyes that zoom in to the scale of the individual women and men affected by war and that have seen its moral dimension. In the 5th century BCE the Chinese philosopher Mozi, the founder of a rival religion to Confucianism and Taoism, noted:

For most of human history, the justification for war was pithily captured by Julius Caesar: "I came. I saw. I conquered." Conquest was what governments did. Empires rose, empires fell, entire populations were annihilated or enslaved, and no one seemed to think there was anything wrong with it. The historical figures who earned the honorific "So-and-So the Great" were not great artists, scholars, doctors, or inventors, people who enhanced human happiness or wisdom. They were dictators who conquered large swaths of territory and the people in them. If Hitler's luck had held out a bit longer, he probably would have gone down in history as Adolf the Great. Even today the standard histories of war teach the reader a great deal about horses and armor and gunpowder but give only the vaguest sense that immense numbers of people were killed and maimed in these extravaganzas.

The occasional Western seer too paid homage to the ideal of peace. The prophet Isaiah expressed the hope that "they shall beat their swords into plowshares, and their spears into pruning hooks: nation shall not lift up sword against nation, neither shall they learn war any more." Jesus preached, "Love your enemies, do good to those who hate you, bless those who curse you, pray for those who mistreat you. If someone strikes you on one cheek, turn to him the other also." Though Christianity began as a pacifist movement, things went downhill in 312 CE when the Roman ruler Constantine had a vision of a flaming cross in the sky with the words "In this sign thou shalt conquer" and converted the Roman Empire to this militant version of the faith.

If a man on seeing a little black were to say it is black, but on seeing a lot of black were to say it is white, it would be clear that such a man could not distinguish black and white… So those who recognize a small crime as such, but do not recognize the wickedness of the greatest crime of all -- the waging of war on another state -- but actually praise it -- cannot distinguish right and wrong.

To kill one man is to be guilty of a capital crime, to kill ten men is to increase the guilt ten-fold, to kill a hundred men is to increase it a hundred-fold. This the rulers of the earth all recognize, and yet when it comes to the greatest crime -- waging war on another state -- they praise it!…

The moral arguments against war are irrefutable. As the musician Edwin Starr put it, "War. Hunh! What is it good for? Absolutely nothing. War means tears to thousands of mothers' eyes, when their sons go to fight and lose their lives." But for most of history this argument has not caught on, for two reasons.

Periodic expressions of pacifism or war-weariness over the next millennium did nothing to stop the nearly constant state of warfare. According to the Encyclopaedia Britannica, the premises of international law during the Middle Ages were as follows: "In the absence of an agreed state of truce or peace, war was the basic state of international relations even between independent Christian communities; (2) Unless exceptions were made by means of individual safe conduct or treaty, rulers saw themselves entitled to treat foreigners at their absolute discretion; (3) The high seas were no-man's-land, where anyone might do as he pleased." In the 15th, 16th, and 17th centuries, wars broke out between European countries at a rate of about three new wars a year.

Pacifism is also vulnerable to militaristic forces within a country. When a country is embroiled in a war or on the verge of one, its leaders have trouble distinguishing a pacifist from a coward or a traitor. The Anabaptists are one of many pacifist sects that have been persecuted throughout history.

One is the other-guy problem. If a nation decides not to learn war anymore, but its neighbor continues to do so, its pruning hooks will be no match for the neighbor's spears, and it may find itself at the wrong end of an invading army. This was the fate of Carthage against the Romans, India against Muslim invaders, the Cathars against the French and the Catholic Church, and the various countries stuck between Germany and Russia at many times in their history.

To gain traction, antiwar sentiments have to infect many constituencies at the same time. And they have to be grounded in economic and political institutions, so that the war-averse outlook doesn't depend on everyone's deciding to become and stay virtuous. It was in the Age of Reason and the Enlightenment that pacifism evolved from a pious but ineffectual sentiment to a movement with a practicable agenda.

'Tis not due yet: I would be loath to pay him before his day. What need I be so forward with him that calls not on me? Well, 'tis no matter; honour pricks me on. Yea, but how if honour prick me off when I come on? How then? Can honour set to a leg? No. Or an arm? No. Or take away the grief of a wound? No. Honour hath no skill in surgery then? No. What is honour? A word. What is that word honour? Air -- a trim reckoning! Who hath it? He that died a Wednesday. Doth he feel it? No. Doth he hear it? No. 'Tis insensible then? Yea, to the dead. But will it not live with the living? No. Why? Detraction will not suffer it. Therefore I'll none of it. Honour is a mere scutcheon -- and so ends my catechism.

One way to drive home the futility and evil of war is to tap the distancing power of satire. A moralizer can be mocked, a polemicist can be silenced, but a satirist can get the same point across through stealth. By luring an audience into taking the perspective of an outsider -- a fool, a foreigner, a traveler -- a satirist can make them appreciate the hypocrisy of their own society and the flaws in human nature that foster it. If the audience gets the joke, if the readers or viewers lose themselves in the work, they have tacitly acceded to the author's deconstruction of a norm without anyone having had to rebuff it in so many words. Shakespeare's Falstaff, for example, delivers the finest analysis ever expressed of the concept of honor, the source of so much violence over the course of human history. Prince Hal has urged him into battle, saying "Thou owest God a death." Falstaff muses:

Detraction will not suffer it! More than a century later, in 1759, Samuel Johnson imagined a Quebec Indian chief commenting on "the art and regularity of European war" in a speech to his people during the Seven Years' War:

They have a written law among them, of which they boast as derived from him who made the earth and sea, and by which they profess to believe that man will be made happy when life shall forsake him. Why is not this law communicated to us? It is concealed because it is violated. For how can they preach it to an Indian nation, when I am told that one of its first precepts forbids them to do to others what they would not that others should do to them…

The sons of rapacity have now drawn their swords upon each other, and referred their claims to the decision of war; let us look unconcerned upon the slaughter, and remember that the death of every European delivers the country from a tyrant and a robber; for what is the claim of either nation, but the claim of the vulture to the leveret, of the tiger to the fawn?

He was perfectly astonished with the historical Account I gave him of our Affairs during the last Century, protesting it was only a Heap of Conspiracies, Rebellions, Murders, Massacres, Revolutions, Banishments, the very worst Effects that Avarice, Faction, Hypocrisy, Perfidiousness, Cruelty, Rage, Madness, Hatred, Envy, Lust, Malice, or Ambition could produce…

(A leveret is a young hare.) Jonathan Swift's Gulliver's Travels (1726) was the quintessential exercise in the shifting of vantage points, in this case from the Lilliputian to the Brobdingnagian. Swift has Gulliver describe the recent history of his homeland to the King of Brobdingnag:

"As for yourself," (continued the King), "who have spent the greatest Part of your Life in Travelling, I am well disposed to hope you may hitherto have escaped many Vices of your Country. But by what I have gathered from your own Relation, and the Answers I have with much Pain wringed and extorted from you, I cannot but conclude the Bulk of your Natives to be the most pernicious Race of little odious Vermin that Nature ever suffered to crawl upon the Surface of the Earth."

Together with satires suggesting that war was hypocritical and contemptible, the 18th century saw the appearance of theories holding that it was irrational and avoidable. One of the foremost was gentle commerce, the theory that the positive-sum payoff of trade should be more appealing than the zero-sum or negative-sum payoff of war. Though the mathematics of game theory would not be available for another two hundred years, the key idea could be stated easily enough in words: Why spend money and blood to invade a country and plunder its treasure when you can just buy it from them at less expense and sell them some of your own? The Abbé de Saint Pierre (1713), Montesquieu (1748), Adam Smith (1776), George Washington (1788), and Immanuel Kant (1795) were some of the writers who extolled free trade because it yoked the material interests of nations and thus encouraged them to value one another's well-being. As Kant put it, "The spirit of commerce sooner or later takes hold of every people, and it cannot exist side by side with war… Thus states find themselves compelled to promote the noble cause of peace, though not exactly from motives of morality."

Satires appeared in France as well. In one of his pensées, Blaise Pascal (1623-62) imagined the following dialogue: "Why are you killing me for your own benefit? I am unarmed." "Why, do you not live on the other side of the water? My friend, if you lived on this side, I should be a murderer, but since you live on the other side, I am a hero, and it is just." Voltaire's Candide (1759) was another novel that slipped scathing antiwar commentary into the mouth of a fictitious character, such as the following definition of war: "A million assassins in uniform, roaming from one end of Europe to the other, murder and pillage with discipline in order to earn their daily bread."

As they did with slavery, Quakers founded activist groups that opposed the institution of war. Though the sect's commitment to nonviolence sprang from its religious belief that God speaks through individual human lives, it didn't hurt the cause that they were influential businessmen rather than ascetic Luddites, having founded, among other concerns, Lloyd's of London, Barclays Bank, and the colony of Pennsylvania.

The most remarkable antiwar document of the era was Kant's 1795 essay "Perpetual Peace." Kant was no dreamer; he began the essay with the self-deprecating confession that he took the title of his essay from the caption on an innkeeper's sign with a picture of a burial ground. He then laid out six preliminary steps toward perpetual peace, followed by three sweeping principles. The preliminary steps were that peace treaties should not leave open the option of war; that states should not absorb other states; that standing armies should be abolished; that governments should not borrow to finance wars; that a state should not interfere in the internal governance of another state; and that in war, states should avoid tactics that would undermine confidence in a future peace, such as assassinations, poisonings, and incitements to treason.

He then outlined his three conditions for perpetual peace. The first is that states should be democratic. Kant himself preferred the term republican, because he associated the word democracy with mob rule; what he had in mind was a government dedicated to freedom, equality, and the rule of law. Democracies are unlikely to fight each other, Kant argued, for two reasons. One is that a democracy is a form of government that by design ("having sprung from the pure source of the concept of law") is built around nonviolence. A democratic government wields its power only to safeguard the rights of its citizens. Democracies, Kant reasoned, are apt to externalize this principle to their dealings with other nations, who are no more deserving of domination by force than are their own citizens.

More interesting were his "definitive articles." Kant was a strong believer in human nature; elsewhere he had written that "from the crooked timber of humanity no truly straight thing can be made." Thus he began from a Hobbesian premise:

The state of peace among men living side by side is not the natural state; the natural state is one of war. This does not always mean open hostilities, but at least an unceasing threat of war. A state of peace, therefore, must be established, for in order to be secured against hostility it is not sufficient that hostilities simply be not committed; and, unless this security is pledged to each by his neighbor (a thing that can occur only in a civil state), each may treat his neighbor, from whom he demands this security, as an enemy.

More important, democracies tend to avoid wars because the benefits of war go to a country's leaders whereas the costs are paid by its citizens. In an autocracy "a declaration of war is the easiest thing in the world to decide upon, because war does not require of the ruler, who is the proprietor and not a member of the state, the least sacrifice of the pleasures of his table, the chase, his country houses, his court functions, and the like. He may, therefore, resolve on war as on a pleasure party for the most trivial reasons." But if the citizens are in charge, they will think twice about wasting their own money and blood on a foolish foreign adventure.

Kant's second condition for perpetual peace was that "the law of nations shall be founded on a Federation of Free States"-- a "League of Nations," as he also called it. This federation, a kind of international Leviathan, would provide objective, third-party adjudication of disputes, circumventing every nation's tendency to believe that it is always in the right. Just as individuals accede to a social contract in which they surrender some of their freedom to the state to escape the nastiness of anarchy, so it should be with states: "For states in their relation to each other, there cannot be any reasonable way out of the lawless condition which entails only war except that they, like individual men, should give up their savage (lawless) freedom, adjust themselves to the constraints of public law, and thus establish a continuously growing state consisting of various nations which will ultimately include all the nations of the world."

Kant didn't have in mind a world government with a global army. He thought that international laws could be self-enforcing. "The homage which each state pays (at least in words) to the concept of law proves that there is slumbering in man an even greater moral disposition to become master of the evil principle in himself (which he cannot disclaim) and to hope for the same from others." The author of "Perpetual Peace" was, after all, the same man who proposed the Categorical Imperative, which stated that people should act so that the maxim of their action can be universalized. This is all starting to sound a bit starry-eyed, but Kant brought the idea back to earth by tying it to the spread of democracy. Each of two democracies can recognize the validity of the principles that govern the other. That sets them apart from theocracies, which are based on parochial faiths, and from autocracies, which are based on clans, dynasties, or charismatic leaders. In other words, if one state has reason to believe that a neighboring one organizes its political affairs in the same way that it does because both have stumbled upon the same solution to the problem of government, then neither has to worry about the other one attacking, neither will be tempted to attack the other in preemptive self-defense, and so on, freeing everyone from the Hobbesian trap. Today, for example, the Swedes don't stay up at night worrying that their neighbors are hatching plans for Norway Über Alles, or vice versa.

The third condition for perpetual peace is "universal hospitality" or "world citizenship." People from one country should be free to live in safety in others, as long as they don't bring an army in with them. The hope is that communication, trade, and other "peaceable relations" across national boundaries will knit the world's people into a single community, so that a "violation of rights in one place is felt throughout the world."

Obviously the satirists' deglorification of war and Kant's practical ideas on how to reduce it did not catch on widely enough to spare Western civilization the catastrophes of the next century and a half. But as we shall see, they planted the seeds of a movement that would blossom later and turn the world away from war. The new attitudes had an immediate impact as well. Historians have noted a change in the attitudes to war beginning around 1700. Leaders began to profess their love of peace and to claim that war had been forced upon them. As Mueller notes, "No longer was it possible simply and honestly to proclaim like Julius Caesar, 'I came, I saw, I conquered.' Gradually this was changed to 'I came, I saw, he attacked me while I was just standing there looking, I won.' This might be seen as progress."

More tangible progress was seen in the dwindling appeal of imperial power. In the 18th century some of the world's most bellicose nations, such as the Netherlands, Sweden, Spain, Denmark, and Portugal, reacted to military disappointments not by doubling down and plotting a return to glory but by dropping out of the conquest game, leaving war and empire to other countries and becoming commercial nations instead. One of the results, as we shall see in the next chapter, was that wars between great powers became shorter, less frequent, and limited to fewer countries (though the advance of military organization meant that the wars that did occur were more damaging).

And the greatest progress was yet to come. The extraordinary decline of major war in the last sixty years may be a delayed vindication of the ivory-tower theories of Immanuel Kant -- if not "perpetual peace," then certainly a "long peace," and one that keeps getting longer. As the great thinkers of the Enlightenment predicted, we owe this peace not just to the belittling of war but to the spread of democracy, the expansion of trade and commerce, and the growth of international organizations.