TODAY THE practice as well as the theory of foreign policy is divided between the traditions of liberalism and realism (or realpolitik). Ever since the UN Universal Declaration of Human Rights in 1948, international politics has been influenced by the discourse of “human rights,” rooted in the tradition of natural-rights philosophy that dates back to the seventeenth-century British philosopher John Locke. The idea of human or natural rights is commonly identified with the liberal tradition in foreign affairs. The liberal tradition, favoring international organization, international law, free trade and national self-determination, is often identified with Locke as well as with Adam Smith and Woodrow Wilson.

Liberalism is often contrasted with realism, the tradition identified with practitioners of power politics like Otto von Bismarck and Henry Kissinger and theorists like Hans Morgenthau. Realism is divided among several schools, but most realists agree that the central fact of world politics is the competition for relative power among sovereign states in a condition of global anarchy.

In the actual practice of contemporary statecraft, the two traditions are usually combined. Most countries, including the United States, the most powerful state in the system, mix power politics with moral and legal arguments in their foreign policies. But this commingling of realism and liberalism is expedient, rather than principled. It does not rest on any coherent moral and political philosophy in which elements from the realist and liberal tradition can find appropriate places. Attempts to produce a synthesis, a “realist liberalism” or a “liberal realism,” seem more forced than natural.

This dilemma makes John Locke’s thinking about world politics particularly interesting. Although he is usually classified as a liberal, Locke’s natural-rights theory frequently leads to conclusions closer to modern realism than liberalism. In an era when the stale debate between liberals and realists often frustrates foreign-policy practitioners and thinkers alike, Locke’s unusual combination of realist and liberal themes bears surprising relevance to contemporary debates about human rights, power politics and world order.

ALTHOUGH HE was careful never to mention his notorious precursor Thomas Hobbes, John Locke followed Hobbes in rooting social and political order in the natural rights of human beings in a “state of nature.” Chief among these natural rights is the right of self-preservation of life, a right aided by the auxiliary natural rights of liberty or “property,” a term used broadly to mean the right of individuals to labor for their own subsistence. Hobbes’s description of the state of nature is well known:

“In such condition there is no place for industry, because the fruit thereof is uncertain: and consequently no culture of the earth; no navigation, nor use of the commodities that may be imported by sea; no commodious building; no instruments of moving and removing such things as require much force; no knowledge of the face of the earth; no account of time; no arts; no letters; no society; and which is worst of all, continual fear, and danger of violent death; and the life of man, solitary, poor, nasty, brutish, and short.”

Unlike Hobbes, Locke distinguishes between the state of nature and the state of war. According to Locke, moral norms exist even in a prepolitical condition of anarchy. Individuals have a right to defend themselves and take what they need to survive from nature by their own labor, but they have no right to kill or enslave others, except in self-defense. Each individual possesses the “executive power” necessary to enforce the law of nature, in their own defense and that of others. When they form a community by means of a hypothetical (not historical) social contract, the members of the community delegate their power to enforce the law of nature—that is, to preserve their lives, liberties and property—to the government. But the contract is among the members of the community, not between the community and the state, which is merely the agent of the sovereign people, who delegate limited powers to it as a trust that they can revoke at any time.

Although Locke differed from Hobbes in asserting that a minimal morality precedes community, his depiction of the prepolitical condition is similar to Hobbes’s famous description of the state of nature as “nasty, brutish and short.” Locke’s state of nature is not in itself a state of perpetual war, as it is for Hobbes. But Locke emphasizes that the absence of settled laws enforced by government make the state of nature one of “disorder,” “uncertainty” and “anarchy,” from which individuals flee to the “sanctuary” of a commonwealth created by a social compact. Defense against outsiders and criminals is the chief purpose of a state:

“Political power, then, I take to be a right of making laws with penalties of death and, consequently, all less penalties for the regulating and preserving of property, and of employing the force of the community in the execution of such laws, and in the defence of the commonwealth from foreign injury, and all this only for the public good.”

The Lockean commonwealth exists primarily to defend the community against violence from without and within.

IN HIS hostility to imperialism and colonialism, as well as in his solicitude for the rights of conquered peoples, Locke is closer to the liberal tradition that invokes his name than to versions of realpolitik that dismiss attempts to limit the legitimate goals and methods of war as sentimental and utopian. Unlike many liberals, however, Locke does not base his opposition to wars of conquest and annexation—or his ideas about the proper treatment of defeated enemies—on humanitarian sentiments or moral duties that transcend borders. Instead, his arguments about these topics are rigorously derived from his specific claims about the limited ends of legitimate governments.

In his study Locke on War and Peace, Richard H. Cox points out that Locke’s limitation of legitimate war to the defense of the lives, liberties and estates of a state’s own citizens means that wars of glory or conversion are illegitimate by definition:

“The individual’s right to make war is given up to the commonwealth with the express limitation that it shall be employed only “in the defence of the commonwealth from foreign injury,” which is to say only for “the public good.” Consequently, rulers who use the war power only to achieve personal glory and power through conquests . . . have broken the fundamental law of the commonwealth, violated the law of nature regarding the preservation of their own subjects, and exposed themselves to punishment and removal from office. . . . Finally, the public force of the commonwealth can never legitimately be used to instigate a war on religious grounds, such as in an attempt to stamp out heresy and idolatry. . . . Since a war waged for any of the preceding causes is fundamentally unjust according to the law of nature, the aggressor, if victorious, acquires no rights whatever, under the law, over the victim of the conquest.”

The same notions of natural rights and social contracts limit what victors in a just, defensive war can legitimately do to their defeated enemies. While the victors may punish the foreign leaders who launched an aggressive war, they cannot engage in collective punishment. Locke’s reasoning is subtle and somewhat paradoxical. Remember, according to Locke, a state’s powers are delegated by its people and limited to a few (not all) of the powers that each individual possesses in the state of nature. The individual’s power of self-defense, transferred by the members of a particular community to the government, becomes the legitimate power of waging war for the defense of the community. But if individuals in the state of nature have no right to murder, rob or enslave innocents without violating the law of nature, then they cannot delegate a power to murder, rob and enslave foreigners to their government.

It follows, then, that the leaders of a foreign country that has engaged in an aggressive war must be assumed to have been acting ultra vires, that is, to have violated their duties as agents of their own people. (Living in an era of dynastic monarchies and patrician city-republics following the age of European barbarian invasions and preceding the age of mass politics, Locke says nothing about cases in which entire populations support aggressive wars.) The victorious defenders of the state that was attacked can punish the foreign leaders and exact reparations for the damage they have suffered, but if possible, says Locke, the reparations should not lead to the starvation of the defeated population. Furthermore, the victor in a defensive war cannot annex enemy territory, which still belongs to the people of the former enemy state. And if the enemy government has been destroyed, the victors cannot install a new one. The people of the defeated country must organize a new government themselves.

The sole exception Locke makes to his principled opposition to imperialism is that productive populations are entitled to appropriate and develop “waste” land, if it is left undeveloped by primitive nomads or others who make only light use of it. No environmentalist, Locke considers nature that has not yet been transformed for human use to be worthless. Locke argues that such relatively empty and undeveloped territory is, in essence, still part of the global commons which individuals and commonwealths can appropriate for their own uses. This theory was used by Anglo-Americans to justify the colonization of thinly inhabited North America, Australia and New Zealand by settlers at the expense of their aboriginal peoples. But it could not be used to justify the rule of the Spanish over dense Indian populations in Central and South America or the later rule of the British empire over the inhabitants of India and other sedentary, agrarian populations.