Deborah Pearlstein is a professor of constitutional and international law at Cardozo School of Law, Yeshiva University.

With their volleying threats of mutual destruction in recent weeks, neither Kim Jong Un nor President Trump has seemed much concerned that international law prohibits states from starting wars. From what we know of these two leaders, we should not be terribly surprised. North Korea is a serial violator of United Nations Security Council resolutions aimed at stemming its nuclear and missile tests. Trump was elected to the presidency having called into question the United States’ engagements with NATO, the North American Free Trade Agreement, the Paris climate accord, the Iran nuclear deal and the ­Trans-Pacific Partnership, among others. The crisis on the Korean Peninsula follows a string of controversies that have strained the most basic rules of the international order: Russia’s invasion of Ukraine, China’s assertion of sovereignty over its homemade islands in the South China Sea and Syria’s use of long-banned chemical weapons.

This seems an especially difficult moment to make a persuasive case that the international move in the last century to outlaw war “has largely, if not perfectly, worked.” But it is exactly the argument that Yale Law School professors Oona Hathaway and Scott Shapiro put forward in their new book. “The Internationalists” is partly a rich history of the emergence of the modern international legal order and partly an empirical study of the change in war the authors say the law helped produce.

As a legal history, the book is indispensable. It traces the intellectual origins of today’s international legal order to the much-maligned Paris peace pact of 1928 — the agreement in which the United States and 14 other nations renounced “recourse to war” as an “instrument of national policy,” only to embark upon the most destructive war in world history scarcely more than a decade later. Indeed, a version of the pact’s rule would not be formally codified into law until the 1945 adoption of the U.N. Charter, a treaty to which the United States and every other nation in the world is party today, and which prohibits the threat or use of force “against the territorial integrity or political independence of any state.” But as Hathaway and Shapiro demonstrate in richly researched detail, the watershed moment in legal thought was contained in the 1928 idea: that it was time to abandon the “old world order” in which war was not only a common but also a lawful means of resolving disputes. When war was legal, non-warring states could not impose economic sanctions on an aggressor state without risking their own status as neutral parties; diplomacy was regularly conducted at the point of a gun; and territory belonged to the side whose weapons were best. The move to outlaw war carried logical — and in time actual — consequences for all of these old customs. States that started wars in the new world were now violating the law and could thus be punished by other states economically or even criminally for their transgressions. Agreements concluded under threat of force could no longer be valid. And while aggressor states, of course, still had the power to conquer territory by force, that territory would no longer be recognized as legally theirs.

“The Internationalists,” by Oona Hathaway and Scott Shapiro (Simon & Schuster)

Among the greatest strengths of “The Internationalists” is the authors’ acute attention to the role of individual thinkers, some long overlooked, and to the impact of happenstance — the ways, for instance, an idealist’s memo could land in a pragmatist’s hands. So it was, for example, that Salmon Levinson, son of German immigrants and Yale Law School graduate, was able to persuade his former Yale classmate and Secretary of State Henry Stimson to proclaim in 1932 that the United States would no longer recognize any international conquest or agreement “brought about by means contrary to” the peace pact. The United States did not join the League of Nations, but with Stimson’s engagement it agreed to work with league members to deny Japan the economic benefit of its conquest of Manchuria. Even in great-power politics, personalities matter.

It is a shame the authors don’t focus as strongly on personalities in the book’s chapters on how the law changed behavior. Instead, they turn their attention to demonstrating statistically that interstate wars resulting in lasting territorial conquest became dramatically less significant after the 1928 peace pact. This shift was not, the authors stress, because interstate war dropped off dramatically after 1928 — that decline did not begin until after World War II. Rather, they argue, it was because most territory taken in wars after 1928 was later returned to its pre-1928 owners or, if not, was far smaller in area than the vast land conquests of the old world order.

Yet even accepting the finding that the nature of land claims changed after 1928, it remains far from clear that this shift in state behavior came about in some measure because of the prewar revolution in law the authors so ably describe. As they acknowledge, the advent of nuclear weapons and the spread of democracy and trade may also help explain the change in state behavior in the latter half of the 20th century. Without greater insight into, for example, the internal deliberations among officials in the United States, Britain and France after World War II, it is difficult to know for sure whether any decision-maker would point to the pact of 1928 — or the legal developments it prompted — as influencing their decision in 1945 to claim no important new territory for themselves.

Understanding whether law makes a difference in states’ behavior is a notoriously difficult problem. Governments are complex organisms, led by individuals who regularly reach decisions after consultation with sometimes warring casts of advisers, each one with his or her own reasons for urging a particular action, reasons that can include strategy, morality and law (or none of these) at the same time. In such circumstances, looking at what states do — whether they follow the law or not — can tell us only a little about why they do it.

“The Internationalists” provides a great service in illustrating the ways in which law can speak powerfully to individual decision-makers. An even greater service would be to show us more about what kind of decision-maker it takes to listen.

The Internationalists
How a Radical Plan to Outlaw War Remade the World

By Oona A. Hathaway and Scott J. Shapiro

Simon & Schuster. 581 pp. $30