In 1928, the leaders of 15 countries committed to renouncing war as a tool for resolving international disputes. They enshrined this commitment in the Kellogg-Briand Pact (sometimes referred to as the Paris Peace Pact) and were later joined by 47 other countries. But war, of course, continued, and the pact is generally remembered as a well-meaning but ineffectual fantasy—when it is remembered at all.  

Now, Yale law professors Oona Hathaway and Scott Shapiro, the authors of The Internationalists: How a Radical Plan to Outlaw War Remade the World, seek to reinterpret the pact’s historical significance. Their book argues that it’s because war was made illegal in 1928 that nations rarely go to war with each other anymore (though wars within states are another matter).

Critics have taken issue with this “ambitious causal claim,” noting that the pact is far from the only explanation for the decline in wars between states. What about the aversion to conflict brought about by the horrors of World War I and World War II? Or the development of nuclear weapons as a tool of deterrence? Or the rise of the United States as the dominant world power? Or the increasing economic interdependence between states?

Although their thesis is controversial, the authors of The Internationalists have revived an often-forgotten episode in political history—and its implications are urgent. “We see lots of signs that we are once again at a tipping point in the world,” Hathaway told me. I spoke with her and Shapiro about the lessons to be drawn from the pact today. Below is a condensed and edited transcript of our conversation.

Annabelle Timsit: The premise of your book is that the Pact outlawed war, marking a shift from an Old to a New World Order. Can you talk about what that means in today’s context?

Oona Hathaway: The book argues that the countries of the world succeeded in outlawing war in 1928, but outlawing war is not the same thing as ending war. … Much of the story that we tell is the way in which that set in motion the gradual construction of a New World Order, built around the idea that states couldn’t just go to war for whatever purpose they desired; that, in fact, aggressive war was no longer allowed and that they had to find other ways to resolve their disputes. That moment of outlawry of war, though it didn’t succeed in ending war, set the stage for everything that followed, and in particular for the UN Charter and for the modern legal order.

Timsit: You consider the rise of intrastate conflict [civil wars] a tradeoff for the large decrease in the number of interstate conflict [wars between countries] in the New World Order. Do you think one is worse than the other?

Hathaway: First of all, no war is good. It is the case that, historically, wars between states have led to a lot more people dying, because states tend to have more powerful [militaries] and machinery. That doesn’t mean that we shouldn’t be deeply concerned about the rise of civil war. And in fact, part of what we want to say is that there are downsides to the international legal system that we have today.

The upside is that there are significantly fewer wars between states; that conquest has effectively come to an end (with rare exceptions); and that we have unprecedented prosperity around the globe. The downside is that we see a rise in conflict within states, and part of the reason for that is the fact that we’ve outlawed war between states. That is a dark side of the modern legal order, but in our view, it’s not a reason to throw out the system—there’s much that we can do to address the scourge of intrastate war, including by building capacity for states to effectively govern themselves.

Timsit: The modern legal order didn’t prevent, say, the 2003 Iraq War. So has the system begun to fail, and is the Old World Order already returning?

Hathaway: We end the book with a warning that we once again find ourselves at a turning point in history. The postwar consensus on the illegality of war is under greater assault today than it has been in seven decades. There are many reasons for this, including the threat from ISIS and Russia’s willingness to flaunt the global legal order (in particular, its seizure of Crimea and efforts to foment unrest in Ukraine and Georgia). Meanwhile, China’s attempts to assert sovereignty over disputed rocks and islands in the South China Sea, Iran’s support for terrorist groups, and North Korea’s repeated threats of military force against its neighbors and others are also deeply corrosive. And, as you point out, the testing of the system is not entirely new. The decision of the United States and its allies to use force against Iraq in 2003 under a legal justification widely viewed as tenuous at best, and deeply cynical and disingenuous at worst, dealt a terrible blow to the legal order and weakened the system even before these more recent challenges arose.

But the test of the international legal system is not whether we can point to instances where the law was violated. We should instead look to whether the law has largely, if not perfectly, worked. To that the answer is clearly “yes.”

Timsit: Your book’s thesis that law alone has stopped war is controversial. What would you respond to realist critics of your book, who say it’s really just power that matters?

Hathaway: Realists fail to understand how law works. … When it is most effective, the law does not induce states to act contrary to incentives; it changes those incentives themselves. To take one example: After war was outlawed, conquest was no longer legal. As a result, when Japan invaded Manchuria in 1931, the U.S. and the states that were party to the League of Nations refused to recognize the conquest, pointing specifically to Japan’s violation of its legal obligations under the Pact, which Japan had ratified. This change in the rules thus changed the incentives states faced—they could still seize land with force, but they could no longer enjoy the fruits of their conquests.

The realist might respond that, even if the change in the law changes behavior, that doesn’t prove that law matters: The law is simply a tool of the powerful—great nations create law that is in their interests, and when the law changes behavior, power is doing the causal work, not the law. But to say that the powerful shape the law to reflect their interests is not to say that law is merely a byproduct of power. 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.

Scott Shapiro: So much of our book is an attempt to show that the law is playing such a strong role in the way states behave that we don’t even notice it. It’s hiding in plain sight. So it’s kind of a triumph of the outlawry of war that we don’t even recognize it at work. But, even though we’re lawyers and think law is really useful, we also think that the law has limits, that is, that there’s not always a legal answer to all the questions that arise. When that happens, there are arbitral bodies [such as] the International Court of Justice.

But let me also back up by saying that, in some sense, the challenge of the modern global order is the fact that states no longer have the tool to respond to violations of the law with war. War was a tool of law enforcement before 1928. And so really, the question that the internationalists had to wrestle with, and states had to wrestle with, was: How do we live without war? If we renounce war as a way of resolving our disputes, how do we respond to violations of international law?

Timsit: What’s the answer to those questions?

Shapiro: When Russia annexed Crimea, what would have been an appropriate, non-violent response in that case? We developed this idea in the book of “outcasting”—that is, in a world in which war is no longer a legitimate way of enforcing the law, states outcast the rule-violator, they refuse to do something with the rule-violator. In the case of Crimea, extensive sanctions were placed on Russia and on Russian officials to make their conquest and annexation of Crimea painful. Most of the world does not recognize Crimea as being part of the Russian Federation, and consequently airplanes don’t fly there directly, credit cards don’t work there, the ATMs don’t work. … Basically, what has happened is that Crimea has been outcasted from the international system. So this is the way in which international law has evolved to live without war as a tool of law enforcement.

Timsit: But what happens when outcasting doesn’t work? Crimea is still in the Russian Federation, not Ukraine.

Hathaway: Sanctions don’t always work immediately; they often take time, they’re not as flashy as an invasion, though they can in the long term actually be more effective. Moreover, the technology of outcasting has improved a lot over the last several decades since it was first deployed in 1931: It has become much more targeted, all the way down to the individuals that are responsible for devising illegal policies that the countries of the world want to reject. And being able to target particular sectors and long-term growth, and in fact, even close off an entire banking sector from the rest of the world, as the world did with Iran, which was key to bringing the Iranians to the negotiating table.

The second thing is that the system does have an escape valve in cases that are truly dangerous. So the UN Charter permits states to act in self-defense under Article 51, and of course the Security Council can authorize interventions. We saw that when Iraq invaded Kuwait. So there are tools under the international legal system that kick in, in cases where outcasting doesn’t succeed, but the hope is that they will only be used where other options aren’t available.

Timsit: What are we supposed to think about the global backlash against this interconnectedness, against globalization, international institutions, and so on? Does international law function without these things?

Hathaway: The law doesn’t work by itself. It does need states and institutions to abide by it, and to work in support of it, and we’re very much concerned that we’re at a moment where that commitment is being questioned and we are seeing real backtracking with regard to commitment to the international legal order. … For all its problems, the international legal order has been remarkably successful in bringing peace and prosperity to the world for the last seven decades and that doesn’t happen on its own, that requires a commitment by states to make the system work. And when that commitment begins to ebb, then the system itself is put at risk. And that’s really the message of the book: Don’t take that for granted.

Shapiro: I also want to emphasize that the book is very much a non-partisan, maybe post-partisan book. It is a lesson that people from both the left and the right need to re-learn, or take to heart, which is that the illegitimacy of war as a way of resolving disputes is central to the modern legal order. Seeing war as the answer to problems is not the province of the right, it’s also a problem on the left, and so the book is trying to speak to both sides of the spectrum.