If you were to ask historians to name the most foolish treaty ever signed, odds are good that they would name the Kellogg-Briand Pact of 1928. The pact, which was joined by 63 nations, outlawed war. Ending war is an absurdly ambitious goal. To think it could be done by treaty? Not just absurd but dangerously naive.

And the critics would seem to be right. Just over a decade later, every nation that had joined the pact, with the exception of Ireland, was at war. Not only did the treaty fail to stop the Second World War but it also failed to stop the Korean War, the Arab-Israeli conflict, the India-Pakistan wars, the Vietnam War, the Yugoslav civil war and the current conflicts in Ukraine, Syria and Yemen.

But the critics are wrong. Though the pact may not have ended all war, it was highly effective in ending the main reason countries had gone to war: conquest. This claim is supported by an empirical analysis we recently conducted of all the known cases of territorial acquisition during military conflict from 1816 to the present.

First, some context. Before 1928, countries had the legal right to wage war. If one state claimed to be victimised by another, international law permitted it to use force to right the wrong. International law also gave countries the right of conquest, meaning they could benefit from war by keeping its spoils, territory and, in some cases, people. The right of conquest did not depend on whether the conqueror was in the right. As long as it claimed to have been a victim, no matter how flimsy its argument, the conqueror became the new legal sovereign. It’s easy to see how the right to wage war could be, and was, abused.

When it outlawed war, the Kellogg-Briand Pact changed nearly every rule that states had followed for centuries. Most important, countries could no longer establish right, justice or title by brute strength. Because war was now illegal, except in cases of self-defence, states lost the right of conquest. Yes, an aggressor could still take a city by force, but doing so would no longer mean that as a matter of law, it became the aggressor’s city. So the Kellogg-Briand Pact changed the law. But did it really change behaviour? Yes, as our data show.

With the research assistance of 18 Yale law students, we found that from 1816 until the Kellogg-Briand Pact was first signed in 1928, there was, on average, approximately one territorial conquest every 10 months. Put another way, the average state during this period had a 1.33 per cent chance of being the victim of conquest in any given year.

At first glance, those may seem like pretty good odds. They are not: A country with a 1.33 per cent annual chance of conquest can expect to be conquered once in an ordinary human lifetime. And these conquests were not small. The average amount of territory seized between 1816 and 1928 was 114,088 square miles per year.

Since the Second World War, conquest has almost come to a full stop. The average number of conquests per year fell drastically — to 0.26 per year, or one every four years. The average size of the territory taken declined to a mere 5,772 square miles per year. And the likelihood that any individual state would suffer a conquest in an average year plummeted — from 1.33 per cent to 0.17 per cent, or once or twice a millennium.

In addition, our data suggest that the two decades after the Kellogg-Briand Pact went into effect — from 1929-1948 — also marked a radical shift in state behaviour. Conquest didn’t stop during this period. But those conquests were almost all reversed. Huge amounts of land that had been seized before 1948 were returned to the countries that had originally held them.

Here’s an even more startling fact: The land returned was not simply territory taken after the beginning of Second World War, in 1939. The reversals went back to a year that predated the war by more than a decade: 1928. So, not only did the US, Britain and France take no new territory for themselves after the war (aside from a small minor adjustment of the border between France and Italy); the Allies also returned Manchuria to China, liberated Ethiopia from Italy and reversed Germany’s conquests throughout Europe.

The striking exception was the Soviet Union, the only one of the Allies to gain any significant territory after the war. Josef Stalin insisted on several territorial gains as the price of peace. But the other Allied powers saw these concessions as regrettable deviations from accepted law, not precedents to be followed in the future.

The illegal annexation of Crimea by Russia in 2014 might seem to prove us wrong. But the seizure of Crimea is the exception that proves the rule, precisely because of how rare conquests are today. Consider that before 1928, the amount of territory conquered every year was equal to roughly 11 Crimeas. In addition, nearly every state in the world has rejected the 2014 annexation as illegal, refusing to recognise Crimea as part of Russia.

By crediting the Kellogg-Briand Pact with changing state behaviour, we do not deny the importance of many other causes that have been offered for the end of conquest and decline of war, such as the advent of nuclear weapons and the considerable rise in free trade. But these explanations are incomplete because they tacitly assume the idea of the pact that might no longer makes right.

We see that idea at work when countries use nuclear weapons to deter aggression rather than to conquer territory. We see that idea at work when countries resort to sanctions instead of bullets, when they recognise — because conquests no longer “stick” — that it’s more profitable to trade than to annex territory.

The missing element in all of these explanations, in other words, is the outlawing of war. Perhaps the Kellogg-Briand Pact wasn’t so foolish, after all.

— New York Times News Service

Oona A. Hathaway and Scott J. Shapiro are professors of law at Yale and the authors of the forthcoming book The Internationalists: How a Radical Plan to Outlaw War Remade the World.