If barbarism persists, then philosophy must protest. If the sword is relentless, then civilization must denounce it.

– Victor Hugo, Oration on Voltaire, 1878

The voice of philosophy has a lot to say about peace, and in the present age we need to hear it more than ever. In a world that is globalized in its economy but still far from cosmopolitan in its outlook, the forces of prejudice, intolerance, and misunderstanding increase tension and generate conflict, both between and within nations. War, or the danger of war, exists at many levels—quite literally: on the ground, where ethnic and religious enmities spill over into violence, and in the upper atmosphere, where the cloud of nuclear war still hangs over us. Philosophers may not be the legislators of the world, but they can help us to clarify moral principles, understand reality, and distinguish true from false knowledge. That is what they are good at. The advice that past philosophers have offered on war and peace is still relevant today.

A group of these were the itinerant Chinese philosophers of the Hundred Schools of Thought, who would sit at the city gate of some small principality during the Era of Warring States (475–221 BC). Their role was to advise the ruler on strategy, such as whether or not to take advantage of a neighbouring state’s weakness and invade. Most of the main Schools—the Confucians, the Mohists, and the Daoists (Taoists) —counselled against war, on both moral and practical grounds. Confucius’s disciple Mengzi (Mencius) warned that wars to capture cities or territory always lead to disaster: they are a way of “teaching the earth how to eat human flesh.” Mohists would cite Mozi (Mo Tzu) himself, who held that states should cooperate for their universal advantage: “If rulers love the states of others as their own, no one will commit aggression.” A Daoist might quote his Master Laozi (Lao Tzu) (Lao: “The ideal relationship between states is one in which they are so close that they can hear their neighbour’s chickens squawk and dogs bark, and yet they leave each other alone.” All these philosophers would urge rulers not be seduced by the rival school of Strategists, who claimed to know the secret of victory.

Nearly two millennia later, in 1516, the great humanist Desiderius Erasmus—whose writings on peace were read by kings and popes, and who was invited to visit the royal courts of England and France—advised the young ruler of the Netherlands that his most important task was to “rule wisely in times of peace” so as to “preclude any future need for the science of war.” His English colleague John Colet preached a sermon for Henry VIII, speaking against war with France.”An early advocate of what we would now call international arbitration, Erasmus argued from reason as well as morality. His most famous essay, The Complaint of Peace (1517), has been described as an effort “to induce men to see a crucial truth—that they were the victims of the tyranny of unsound ideas and corrupt men, and that practical alternatives did indeed exist.”[1]

 Several centuries later, Bertrand Russell reflected in The Ethics of War (1916) on the real causes of the First World War—and how British public opinion was deceived by patriotism and hate. All the great powers of Europe, he pointed out, had precisely the same object: territory, trade, and prestige. The only difference was that the Germans had a lesser share, and wished to increase it; the British wanted to deny them. Both sides wanted total victory, no matter what the cost. “By concentrating attention upon the supposed advantages of the victory of our own side, we become more or less blind to the evils inseparable from war, and equally certain whichever side may ultimately prove victorious,” Russell wrote fearlessly—and was promptly jailed for his views by an outraged British government. He was not the only philosopher to suffer for speaking his mind; but he is almost certainly the only one to be banned from approaching the seashore, for fear he might send signals to German submarines.

Perhaps, in these three examples, it may be said that the philosophers were not actually philosophizing. Mengzi was not reflecting on whether human nature is inherently good or bad; Erasmus was not weighing the balance between free will and predestination; and Russell was not applying himself to mathematical logic (a pursuit he abandoned during the war). They were, however, doing just what philosophers always do: applying their skills to the immediate problems of human existence, of life and death, of war and peace.

Yet there is a tendency for commentators to dismiss the thoughts of philosophers on the subject of peace, as though the thinkers have strayed into an alien area. Immanuel Kant’s Perpetual Peace was long regarded as a marginal essay, a mere indulgence in utopianism, and the piece has only recently received proper attention. The pacific arguments of the Chinese Hundred Schools were long overshadowed by those of the Strategists, and by Sunzi’s (Sun Tzu’s) popular Art of War. Erasmus’s writings on peace are substantial (amounting to more than 400 pages of modern printed text); yet while the works of his more war-minded contemporary Niccolò Machiavelli are available in any good bookshop, it is hard to find a single work by Erasmus—with the possible exception of his In Praise of Folly, the jeu d’esprit he wrote in 1510 to amuse his friend Sir Thomas More.

Similarly neglected are all the other humanitarian peace thinkers, from the Renaissance to the Enlightenment; these include Emeric Crucé, William Penn, the Abbé de Saint-Pierre, and Jean-Jacques Rousseau. Bertrand Russell’s pro-peace arguments in the First World War, and his opposition in later life to the strategy of nuclear deterrence, are portrayed as political causes unrelated to his philosophy. Much the same happened to the peace speeches and poetry of Victor Hugo, and to the pacifist philosophy of Leo Tolstoy.

But one area of philosophical thought that does concern itself with peace (by way of its reverse) is the doctrine of Just War. We may hope that a proper understanding of the conditions that make going to war legitimate, and of the way it may most lawfully be waged, might help to deter the breaking of peace—or at least make the resulting war less inhumane. Still, we should bear in mind that the doctrine has served different purposes in its long history; that it has usually been honoured more in the breach; and that its continued relevance is questionable.

In the early Christian world, the issue of greatest concern was whether Christianity could be reconciled with serving in the Roman imperial army. Though St. Augustine is often described as the Father of Just War, his views changed over a period of forty years; and he himself increasingly urged peace as preferable to war. “It is a higher glory still to stay war itself with a word, than to slay men with the sword,” he wrote, a year before his death. St. Aquinas rationalized the Augustinian doctrine of jus in bello when he and his fellow theologians of the early Middle Ages challenged the temporal rule of the Pope and his bishops. Their answer was the Crusades, when Just War became Holy War.

The theory took a different turn in 17^th-century Europe, when secular nation-states competed with more sophisticated means of warfare such as the use of standing armies and the development of field artillery. The contributions of the Netherlands jurist Hugo Grotius (1583–1645) paved the way for the Enlightenment philosophers, and laid the foundation for modern international law. In his major work De Juri Belli ac Pacis (1625), Grotius looked to natural law to find a way to sanction and restrain war, based on rational and moral principles. Ideally, he saw the international community as governed less by Christian authority than by a framework of treaties and agreements between states. This argument would be carried further by the Swiss philosopher Emmerich de Vattel in his The Law of Nations (1758).

Since that time, the Just War doctrine changed little until quite recently: exposition continued to lean heavily on quotations from Vattel, Grotius, Aquinas, and even Augustine. Its actual effect on the conduct of war is debatable. Kant’s judgement that the doctrine merely provided a fig leaf for aggression—he famously described Vattel’s Law of Nations as a “sorry comforter”—often seems near the truth. Totalitarian regimes have been as likely as Liberal or democratic governments to claim its protection; and both sides in the Crimean,  Boer, and First World wars insisted that theirs was a Just War. In the moralizing ideology of William Gladstone, the use of aggressive force (such as the occupation of Egypt in 1882) could be justified as “force armed with the highest sanction of law.” We may also recall President William McKinley’s justification of the war against Spain in 1898, and the virtual annexation of Cuba in the same year as being “in the name of human progress and civilization.”

More positively, the Just War doctrine provided a basis for humanitarian measures such as the formation of the Red Cross, the development of the Geneva Conventions, and the creation of other international legislation seeking to limit the damage of war. The provisions restricting war in the Covenant of the League of Nations (and later in the Charter of the United Nations) are also based on its concept of justifiability. Just War thinking lay behind much of the argument in the late 1930s supporting collective security, and the fight against fascism and Nazism. Indeed, the Second World War is still viewed as the irrefutable example of a just war—despite the elementary violation (as historian Michael Walzer has observed) that the civilian death toll from “allied terrorism” in that war exceeded half a million men, women, and children.[2]

 In the post–Cold War period, there has been a rebirth of interest in the Just War, and a corresponding decline in the appeal of the “realist” approach that dominated Cold War thinking. From the Gulf War of 1990 onwards, the doctrine has provided an often-contentious yardstick against which wars of intervention or aggression are measured. The Just War concept was invoked by British prime minister Tony Blair to justify the armed intervention in Kosovo (1998-99), and it was often cited by defenders of the US-UK invasion of Iraq (2003). In his Nobel Prize acceptanee speech (2009) President Obama sought to present the US use of force as in the tradition of Just War thinking and the pursuit of a “just peace”.

Whatever the usefulness of Just War theory, we should remind ourselves that philosophizing about war and about peace are two different exercises: one seeks to limit war, the other to prevent it. The peace argument is based not only on morality, but on an acute awareness of the long-term costs of war. Sadly, that awareness is not always shared. In the years leading up to 1914, the main critics of European war—peace advocates such as Jean de Bloch, Norman Angell, Bertha von Suttner, and Jane Addams—predicted, correctly, that such a war would be an economic and social disaster as well as a moral one. These thinkers recognized the fact that peace must be linked to a much broader agenda of justice and development. After the First World War, this philosophy began to be expressed through the economic and social agencies of the new League of Nations. As the opening words of the International Labour Organization’s 1919 Constitution proclaim, “universal and lasting peace can be established only if it is based on social justice.” These values informed the development of the economic and social organizations of the United Nations, and have since become integral to contemporary peace theory.

Part of the difficulty of talking about peace is that the word itself has often been misappropriated. After the Second World War, both superpowers claimed it: Soviet propaganda called Josef Stalin the greatest “fighter for peace,” while the US Strategic Air Command claimed that “peace is our profession.” To speak of peace was to run the risk of being accused of naïveté—or worse, of being a propagandist for the opposing super-power  It was safer to speak of “conflict resolution,” a term chosen by peace-studies pioneer Kenneth Boulding and his colleagues at their research centre at the University of Michigan in 1956. A few lonely voices stillused the “peace” word, including the US peace philosopher John Somerville.[3]

 In 1955, the Russell-Einstein Manifesto—Bertrand Russell’s call for world leaders to ban nuclear weapons, signed by Albert Einstein—was supported by Albert Schweitzer. By the early 1960s , popular protest grew against the superpowers’ policies on nuclear testing, deterrence, and mass destruction. Peace thinkers, including J. D. Bernal, C. Wright Mills, Seymour Melman, Erich Fromm, and Anatol Rapoport, began to reach a wider audience. Vehicles for international peace dialogue included the Pugwash movement, founded by Joseph Rotblat and Bertrand Russell (1957); the Stockholm Institute for Peace Research, founded by Gunnar Myrdal (1966); the Journal of Peace Research, established by Johan Galtung (1964); and the Conference on Peace Research in History (now the Peace History Society, also 1964). In a separate initiative, an international project involving nearly 200 thinkers from 40 countries led to a significant volume of essays on the philosophy of war published in 1969.[4]