Mennonite peace activists in the 1930s were a small part of an international peace movement to promote the Kellogg-Briand treaty. That treaty, passed in 1928, had made war illegal. At Buhler High School in Kansas in 1934-35, the Hi-Y club, led by an intrepid senior student, Elmer Ediger, put up a large Kellogg-Briand poster in front of the school building to promote peace. The Buhler High School principal, Edward E. Kaufman, was a Mennonite pacifist. He had allowed some militant students to put up a pro-war poster. The Buhler Hi-Y antiwar response demonstrated that the debate over the Kellogg-Briand treaty extended to the country’s rural heartland.

The antiwar protesters lost the debate. Though 64 nations had signed the Kellogg-Briand treaty, World War II broke out in Europe in September 1939. The United States entered the war two years later, in December 1941, after Japan attacked the U.S. naval base at Pearl Harbor. The Kellogg-Briand treaty had failed to prevent war.

But was the treaty really a failure?

According to Oona J. Hathaway and Scott J. Shapiro, professors of law at Yale University, the Kellogg-Briand treaty marked a turning point in the legal justification of warfare that diminished nations’ recourse to war in the long run. In The Internationalists: How a Radical Plan to Outlaw War Remade the World, these authors insist on the power of ideas. Critically important was the way nations thought about war. An “old world order” that accommodated aggressive war gave way to a “new world order” that effectively outlawed aggressive war as criminal behavior.

The book begins with a section about the Dutch theorist, Hugo Grotius, who wrote The Law of War and Peace (1625), a comprehensive philosophical analysis that justified war as a permissible response to the violation of natural human rights. Grotius set the legal foundation for the “old world order.” The heroes of this book, however, are peace-minded legal thinkers such as Salmon Levinson, who overturned Grotius’ assumptions, which were once almost universally accepted. Levinson had been influenced by the progressive educator John Dewey, who had supported American involvement in World War I. Levinson and his friends in 1919 created the American Committee for the Outlawry of War. They published a pamphlet calling for war between nations to be declared a “public crime, punishable by the law of nations.” The Internationalist Outlawry of War group became a major force in the wider American peace movement.

The authors do not deny that raw military power matters. They acknowledge that the availability of nuclear weapons helped deter nations from going to war in the latter half of the 20th century. But, they argue, the declining recourse to warfare also resulted from ways of thinking that offered alternatives to the politics of George Kennan, Henry Kissinger and a host of “realistic” diplomats and scholars in the United States and around the world.

Before 1928, the“right of conquest” legally supported the right of nations to make war. The Kellogg-Briand treaty made conquest by brute strength illegal. The successes of nonviolent conflict resolution after World War II were largely achieved by what the authors call “outcasting” — the collective threat to deny benefits to states that refuse to cooperate in nonviolent economic sanctions against the violators of peace.

Hathaway and Shapiro mobilized the research assistance of their law students at Yale to document all cases of international territorial conquest after World War II and the terms in which the conquests were justified. They discovered a dramatic decline of conquests, and a reversal in the results of interstate conquest.

At the same time, Hathaway and Shapiro explain the limits of the “outcasting” technique. Especially sobering is their account of the ideological view of the Islamic State and its plan for a universal caliphate. The intellectual framework of Islamic State thinking was created by the Egyptian radical Sayyid Qutb. It is the polar opposite of “new world order” internationalism and cannot be reconciled with the principles of the Kellogg-Briand treaty.

The claim that the Kellogg-Briand treaty “remade the world” is a startling reversal of commonly accepted images of war and peace in international relations. There are lessons here for the contemporary peace movement, including the Mennonites. It is important for us to view our work for its long-run potential rather than to be discouraged by continuing outbreaks of violence. Peace work is multifaceted in its addressing of local, regional, national and international problems. The movement’s continuing success depends upon its creative thinkers as well as its dedicated activists.

James C. Juhnke is professor emeritus of history at Bethel College in North Newton, Kan.