The League Covenant permitted states to take “such action as they shall consider necessary” if the Council did not reach unanimous agreement. As long as war was still considered a legitimate act of statecraft, Levinson and others argued, national leaders would continue to use it as the expected means of settling differences. The answer was obvious: war must be outlawed. In support of that aim Levinson joined with philosopher John Dewey and others to form the American Committee for the Outlawry of War in 1921, which campaigned on behalf of a treaty prohibiting war.
The seemingly simple idea of outlawing war contained conceptual contradictions. What should be done if states signed the proposed treaty but then ignored its provisions and proceeded with military action against another country? Many argued that the prohibition against war had to include a commitment to apply sanctions against violators, perhaps including collective military action. This was the idea of collective security as embodied in the Covenant. On the other hand if the prohibition was merely a moral principle, with no enforcement mechanism, it would be a meaningless gesture with no practical effect. The logic of the proposal thus pointed toward the need for some form of enforcement mechanism, which meant membership in the League. Many of the members of the committee on outlawry had supported US participation in the League, but they knew that this was not possible politically after the bitter debates of 1919–20. They hoped that outlawry would be a way of achieving the goals of the League by other means.
The outlawry movement emerged at the time when the US government was considering membership in the World Court. Levinson recognized that US participation in the World Court was not realistic and wanted to keep the concept of outlawry separate from debates over US involvement in international institutions. Borah and other Senate isolationists had different ideas. They seized upon the outlawry concept as a way of derailing support for the World Court, favoring a treaty against war that would avoid any entanglements or collective commitments that might lead to war. Peace advocates were caught in a political crossfire. Support for one of their goals, outlawry of war, was used to undermine another, US membership in the World Court. Pacifists and internationalists tried to find a formula that would allow support for both, drafting a “harmony plan” in 1924 for US entry into the World Court on the condition that the Court would outlaw war. Borah and his isolationist colleagues were unpersuaded. They were determined to keep the United States out of any international institution that might imply an obligation to become involved in the military disputes of other nations. Borah joined with other members of the Senate to attach a series of rigid reservations to the World Court proposal, thus effectively scuttling US participation. The only option left for pacifists and internationalists was outlawry, and a watered down and toothless version at that.
Outlawry jumped to the center of international political attention in 1927 when James T. Shotwell, a prominent internationalist who had been an adviser to the Wilson administration, suggested the idea of outlawing war to French Foreign Minister Aristide Briand. The French minister quickly embraced the concept and proposed a US-French treaty that would outlaw war and bind the two sides to the arbitration of disputes. Borah was suspicious of Briand’s bilateral proposal and wanted to avoid an agreement that might obligate the United States to come to the defense of France or any other nation. He convinced Secretary of State Frank Kellogg to negotiate on the basis of the principle “renounce all war or no war at all.” Paris accepted Washington’s demands, and the two sides quickly concluded negotiations for the Treaty on the Renunciation of War, known as the Kellogg–Briand Pact or the Pact of Paris. In August 1928 representatives of dozens of nations, including the United States and the other major powers, gathered for an elaborate ceremony in Paris at which they signed the treaty and solemnly pledged to renounce war as an instrument of national policy.