What is the proper relationship between law and American foreign policy? Can or should law and legal institutions shape international relations? Although Americans vigorously debate these questions today, there was a time when they assumed a tight connection between law and foreign affairs. Lawyers dominated American foreign policy at the turn of the twentieth century; every Secretary of State from 1889 to 1945 was a lawyer, and the Republican Party’s chief foreign policy thinker, Elihu Root, was also its foremost lawyer. In this Article, Professor Jonathan Zasloff argues that this relationship between law and foreign policy had real consequences for the shape of American diplomacy during that period. Professor Zasloff contends that classical legal ideology—the prevailing ideological framework among elite lawyers in the late nineteenth and early twentieth centuries—profoundly influenced the direction of American foreign policy. Classical legal ideology taught that law does not derive its effectiveness from the coercive state but rather from popular custom and social norms. Moreover, it held that law could be effective without state coercion because it stood as a neutral, apolitical source of order that satisfied widely varying social groups. These two beliefs implied that international law could form a basis for a legally regulated world order and that traditional balance-of-power methods were either unnecessary or harmful. As policymakers debated the shape of the post-World War I world order, classical legal ideology told lawyer-statesmen like Root that they did not need to make strategic commitments to ensure global stability. Lawyers imbued with classical legal ideology concentrated on international law and institutions and neglected realpolitik foreign policy. In doing so, they unwittingly contributed to global catastrophe.