The first half of the twentieth century is widely seen as an era when nationalism and war made a mockery of efforts to strengthen international law and peacefully settle disputes. This new take, in contrast, argues that these decades were pivotal for quietly introducing new ideas about international law into state relations. Jones traces the events, personalities, and institutions that marked the era's efforts to bring reason and rule to the prevention of war. The saga begins with the Hague Conferences of 1899 and 1907 and runs through the establishment of the League of Nations and the Permanent Court of International Justice in 1919, the 1928 signing of the Kellogg-Briand Pact, and the founding of the United Nations in 1945. Jones acknowledges that none of these innovations stopped great powers from pursuing their territorial ambitions. But she finds subtle shifts in the way governments legitimated their foreign policies. For example, the Japanese occupation of Manchuria in 1931 was a blow to the League of Nations, but the controversy that followed gave lawyers and diplomats opportunities to articulate principles that would echo in future decades. In this narrative, Jones has identified critical intellectual and political lineage to today's struggles over sovereignty, law, and human rights.