Law is inevitably informed by morality, but it is not the same thing as morality. When we forget this, when we insist that what is wrong must be unlawful, or that it must be lawful to punish every wrong, we undermine the rule of law.

Robert Bolt teaches this lesson memorably in A Man for All Seasons, his play about the life and death of Sir Thomas More. In a key scene, More, then serving as the Lord Chancellor, has just dismissed a young associate, Richard Rich, who has revealed himself to be an informant for More’s political enemies. All of More’s family members present urge him, on various vaguely moral grounds, to arrest Rich; but More, on legal grounds, insists just as strongly that he will not.

More’s daughter Margaret admonishes him that Rich is a “bad” man. More responds that “there’s no law against that.” Roper, More’s son-in-law, observes that being bad is contrary to God’s law. “Then God can arrest him,” More responds, using humor to remind this earnest, not to say fanatical, young man that God’s law and man’s law are not simply the same, and that violation of God’s law is not necessarily grounds for arrest by human political authorities. More’s wife Alice says with disgust: “While you talk, he’s gone.” More’s rejoinder: “And go he should, if he was the Devil himself, until he broke the law.”

This last remark reveals the extent of More’s commitment to the rule of law. Roper is aghast that More would give the “Devil benefit of law,” admitting that he would not only “cut a great road through the law” to get at the devil, but would in fact “cut down every law in England” to do so. More’s beautiful and crushing response is worth quoting in full:

Start your day with Public Discourse

Sign up and get our daily essays sent straight to your inbox.

And when the last law was down, and the Devil turned round on you—where would you hide, Roper, the laws all being flat? This country’s planted thick with laws from coast to coast—man’s laws, not God’s—and if you cut them down—and you’re just the man to do it—do you really think you could stand upright in the winds that would blow then? Yes, I’d give the Devil benefit of law, for my own safety’s sake.

More’s point is simple but essential. Morality tells us what is wrong. Law tells us what is punishably wrong. And everyone’s safety depends on our remembering this distinction and not insisting, going beyond the law, that every wrong must be punished. To do so is to liberate power from the restraints of law.

In an earlier scene, More tells an unprincipled officer of the crown that when statesmen forsake their conscience they “lead their country by a short route to chaos.” For More, nations can arrive at anarchy by two very different paths. A decent public order can be as easily destroyed by either of two opposed, but equally incorrect, dispositions: A cynical indifference to the moral law fosters chaos, but so too does the moralistic manipulation of law.

This seems paradoxical. Morality presumes a fitting public order, so how can bending the law to moral ends result in chaos? For an answer to this question, we might turn to an Englishman of a later generation, John Locke. In his Second Treatise of Civil Government, Locke holds that the moral law, or the natural law, is real. It is part of the order of nature and can be perceived by human reason.

Nevertheless, he adds, it is not usually perceived perfectly, much less obeyed perfectly, by imperfect human beings. Our self-interested prejudices commonly impede the accuracy of our perceptions of, and the strength of our commitments to, justice. This is why we need human laws that prescribe duties and impose prohibitions in unmistakable terms.

It is dangerous to bend the law even to genuinely moral purposes that the law itself does not contemplate, because doing so sets a precedent for bending the law to any purposes that any sufficiently powerful person may desire. The moralistic manipulation of law is in practice indistinguishable from the manipulation of law for the sake of our passions, the regulation of which is the very purpose of the rule of law.

The moralist might respond that no such dangerous precedent is in fact set, because people will always be able to distinguish manipulations of law in a good cause from manipulations of law in a bad cause. This answer is dangerously naive. As Locke reminds us, we only create laws in the first place because the distinction between good and evil, though quite real, is sufficiently obscure in practice that a decent public order cannot be sustained without human law.

All of these considerations, which have been raised in the context of domestic law and domestic order, apply equally to the maintenance of a decent international order. Although international law intends to secure a certain kind of moral order, a just international law requires conceding that not every immoral act is unlawful or punishable.

Among the American founders, Alexander Hamilton was at pains to teach this lesson. In his celebrated Pacificus essays, Hamilton dealt with the practical question whether America was obliged by treaty to enter a war on France’s behalf. At first blush it might seem so, since America had signed a treaty of alliance with France. Hamilton, however, noted that the treaty identified itself more specifically as a treaty of defensive alliance. Therefore, America’s obligation depended on whether the war was on France’s part defensive or offensive.

In making his case, Hamilton reminded his readers that the difference between an offensive and a defensive war is not the same as the difference between a just and an unjust war. Whether a war is just or unjust on the part of a particular nation is almost always a complex moral and political question, often incapable of clear resolution because of the existence of real provocations on both sides. But whether a war is offensive or defensive for a particular nation can be determined more easily: it depends on who started the fighting.

Hamilton’s argument against entering war simply because France’s quest to secure republican government was just is still relevant for international law. The force of international law depends on clear obligations of one country to another. Yet to base those obligations on disputes over the justice, as opposed to the legality, of a given act is to use a perennially murky foundation. In other words, to base such obligations on opinions about justice is to make such obligations infinitely manipulable (and thus, in practice, to abolish them).

Today, America’s foreign policy seems to be guided by this same dangerous moralism. President Obama claims that the Assad regime must be punished, by military force if necessary, for allegedly using chemical weapons in Syria’s civil war. Such an action was surely an atrocity, but was it a crime?

As several commentators have observed, and as even the president has admitted in his public remarks, Syria is not a signatory of the chemical weapons ban. Since international law is a product of treaties, the prohibition was not legally binding on Syria. To adapt More’s language in A Man for All Seasons, Assad may have played the “Devil” but he did not “break the law” by using chemical weapons.

There is a further difficulty. Even supposing that Assad’s use of chemical weapons violated international law, it would not follow that the United States is authorized to punish him. The president has insisted that international norms mean nothing if they are not enforced. There is truth in that remark, but it is equally true that a lawful international order means nothing if any member can arrogate to itself the power to punish infractions.

Lawfulness means not only that unlawful acts are punished, but that they are punished lawfully, by those with proper authority.

Not only does international law not authorize the president of the United States to mete out such punishments, it actually forbids it. Under the United Nations Charter—to which the United States is a signatory, and of which the United States was in fact a chief architect—nations may resort to armed force in self-defense or pursuant to a Security Council resolution. The president can plead neither of these justifications for a bombing campaign and so would be acting not only outside international law but in violation of it if he were to resort to military force against the Assad regime. And he would do so on the moralistic assumption that an atrocity must be unlawful, and must be punishable, and punishable by the president of the United States.

It is easy enough to see that the president’s principles lead “by a short route to chaos.” Undoubtedly, the Syrian rebels fighting against Assad have committed atrocities of their own. Using the president’s reasoning, Russia could claim a right, even a duty, to launch a bombing campaign against them to enforce an international norm against killing non-combatants. Of course, the United States would block a Security Council resolution authorizing such a course. But what would stop Russia from complaining, using the Obama administration’s own moralistic language, that the United States is “holding the Security Council hostage,” and then simply proceeding to do what it wishes? On the basis of what principle could the United States object to such an undertaking?

Admittedly, Russia, fearful of American power, would not dare to do this. Obama’s foreign policy leads “by a short route to chaos” in principle, but the chaos is in fact kept at bay at present by American military, political, and economic might. Muscular liberal moralism works fine, so long as America is far and away the most powerful nation on earth. Will that still be the case, however, twenty years from now, or fifty years from now?

This is a question the defenders of Obama’s moralistic but lawless interventionism should but probably will not ask. They will see no need to ask it because they sense, even if they won’t say it openly, that the Security Council is an anachronism. It establishes, as Hans Morgenthau pointed out, international government by the great powers. It presupposes a set of great powers capable of regulating international conflicts. This is no longer the case. Instead, the world of 2013 features a number of more or less great powers dwarfed by the global superpower, the United States.

Defenders of a merely moral, but not lawful, interventionism are correct that the “great powers” arrangement is an anachronism. But they fail to see that America’s status as the benevolent global hegemon may itself become an anachronism. They forget that in fact all experience suggests that such an uncontested supremacy is fated to pass away at some point, and that when it does, their well-intentioned undermining of international law may be revealed not as wise and noble statesmanship but as dangerous and destabilizing folly.