What Realists Don’t Understand About Law

For centuries, states waged war with one another and conquered one other’s territory with impunity. Today, that is no longer true. Why?

In our recent book The Internationalists, we point to a change in the legal status of war as the core cause of the shift. Realists have reacted with incredulity. Stephen M. Walt, for example, argued in Foreign Policy that if we are right, “I’ve been guilty of educational malpractice for the past 30 years.” He concludes that, thankfully, he can leave his lecture notes intact because we have failed to prove our case.

Walt is far from the only one to think that the act of outlawing war couldn’t possibly change state behavior. If there was a change (and Walt admits there was), then it must be due to something else — the rise of nationalism, the spread of small arms, the growth of alliances, the invention of nuclear weapons — anything but the law. Walt argues that if law really was driving the change in behavior, we “should be able to point to numerous cases where national leaders had a clear incentive to expand their territory and believed it would be easy to do, and then decided not to go ahead either because they believed such an act was inherently wrong or because they were convinced it would never, ever, ever, be accepted by the rest of the international community.”

This reaction reveals a misunderstanding about how law works. When it is most effective, the law doesn’t induce states to act contrary to incentives; it changes those incentives themselves.

To explain why, let us first briefly outline our argument. We argue that before 1928, war was legal. It was, in fact, the central way in which states resolved disputes with one another. War was not a breakdown in the rule of law — war was an instrument of the rule of law. As a result, when states went to war, they got to keep what they caught. And no one — even the losers — could be tried for having waged a war. Killing on the battlefield was completely immune from prosecution in a court of law. Economic sanctions imposed by neutral states on belligerents were illegal. Neutrals were required to treat warring nations with the strictest impartiality. And gunboat diplomacy was legitimate and common.

In 1928, in the Kellogg-Briand Pact, nearly every nation in the world agreed to outlaw war. That set off a cascade of changes that gradually gave shape to the modern legal order. When Japan invaded Manchuria in 1931, the United States and the League of Nations refused to recognize the conquest — announcing the famous “Stimson Doctrine” of nonrecognition grounded expressly in the pact’s renunciation of war. Conquest would no longer be recognized. As a contemporary explained, the aggressor could still take a city by force, “but it would not, as a matter of law, be his city.” Gunboat diplomacy also became illegal — for if waging war is illegal, threatening it must also be illegal. The law of neutrality changed to allow states to show partiality — thus allowing the United States to conclude in 1941 that the Lend Lease-program, which would have been illegal under the old rules, was legal. And, at the end of the war, the Nazis were tried for the crime of aggression — a crime that did not, and indeed could not, exist when war was legal.

These changes all predated the creation of the United Nations. The best way to understand the United Nations, we argue, is as the consolidation and institutionalization of the changes set in motion by the pact. (And, indeed, we show that the very same person who ghostwrote the pact also ghostwrote the first draft of the U.N. Charter and placed the pact at the beginning of the document verbatim.)

Let us return, then, to the Realist objection. If law matters, the Realist argues, we should be able to point to cases where a state had incentives to act — say, to conquer a particular territory — and decided not to act because of the law.

But that’s not how law normally works. At its most powerful, law changes the incentives themselves.

Consider an example: Imagine Mexico owed U.S. citizens millions of dollars and repeatedly failed to repay its debts. No one would say that the United States would have an incentive to invade Mexico and seize territory in repayment of the debt but should hold back only because of the law. Conquering territory in repayment of debts would be an absurd suggestion. Since no one would recognize the conquest, the invasion would have little point. The United States, for example, would not be able to sell Mexico’s natural resources, because buyers would not accept the United States as their true owners. Mexican citizens would likely rebel because the Americans would have no authority over them. The United States might occupy the land, but it would not belong to the United States. Even if the idea were somehow proposed, the response would be that the world would condemn any such act. Not only would the United States not be able to enjoy the fruits of conquest, but it also would be condemned the world over.

Lest one think that the idea is a mere law school hypothetical, think again: In 1848, the United States conquered 500,000 square miles of Mexican territory after Mexico failed to replay $4 million in debts. The rest of the world accepted the United States as the new sovereign over California, Utah, Nevada, and much of the rest of what is now the southwestern part of the country.

Outlawing war thus made conquest, once common, rare. Some states likely refrained from conquests that they thought were in their best interests in order to obey the law. But that was not the primary reason for the shift. Conquest nearly disappeared because the law changed what was in states’ best interests.

The same is true of gunboat diplomacy. When the status of war changed, strong states stopped using their gunboats to coerce weaker states into entering treaties. It may be that some states ended gunboat diplomacy at least in part because the law now prohibited it. But many gave it up because the agreements they could have won at the point of the gun would have been unenforceable and hence useless.

So, yes, the interests of powerful states changed around 1928. It was now in their interests not to recognize conquests, to prosecute leaders for aggression, to use economic power instead of military power to sanction undesirable behavior, and to negotiate agreements. But that was because the law had changed.

The Realist may then respond: If the change in the law is the cause of the change in behavior, that doesn’t prove that the law matters. The law is simply a tool of the powerful — great nations create law that is in their interests. So when the law changes behavior, power is doing the causal work, not the law.

We do not disagree that powerful states drive legal change. In fact, the story we tell is one in which the most powerful states — the United States and France chief among them — drove the decision to outlaw war. They used law self-consciously as an instrument to achieve their interests. After all, as we observe in the book, the states that were most enthusiastic about the pact were those states that had a great deal to gain from ending legal war. Not only had the last military confrontation been brutally destructive, but these states also now dominated substantial swaths of territory, having engaged in legal conquest for centuries. It was convenient for them to call a stop while they were still ahead.

But to say that the powerful shape the law to reflect their interests is not to say that law is merely epiphenomenal. Power may lead to rules, but rules take on lives of their own. They change behavior by changing the incentives for action — not just for the weak but for the strong as well. The same states that favored the change in the law to serve their interests find that their interests are changed by the law. Even if the United States and France wanted to go back to a world in which conquest was legal, they would find it very difficult, if not impossible, to do so. After all, at the close of World War II, with few exceptions, the states that won the war took no new territory for the first time in history.

In short, the law binds nations even if those nations do not care about the law. Since the law changes the incentives that states face, their rational calculations will be affected by the law — whether they like it or not. As long as other states follow the law, it will be in their interests to do so as well.

Realists like to believe that they see the world as it is. But by failing to understand how law operates, they fail to see how the world really works. It may be time for Realists to change their lecture notes after all.

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