American engagement with international law (IL) is regularly criticised as fraught with contradiction and distorted by beliefs in “exceptionalism.” That raises a puzzling question: Why is American international legal policy framed by commitment to the “rule of IL” when this systematically provides a benchmark for challenging the legitimacy of American global power? This thesis argues that the very meaning of the rule of IL is contested according to competing foreign policy ideologies: between American policymakers and their global counterparts, and among American policymakers themselves. Opposition to US legal policy has been structured by forms of “legalism,” as a set of beliefs that law consists of non-instrumental rules, and that the international legal system should be developed by analogy with municipal law. In contrast, American legal policymakers receive IL through competing ideologies that correspond with divisions evident in both legal scholarship and diplomatic history. An internationalist-nationalist jurisdictional dimension intersects with a liberal-illiberal values dimension to form four ideal type conceptions of the rule of IL: Liberal Internationalism, Illiberal Internationalism, Liberal Nationalism and Illiberal Nationalism. These ideal types, including legalism, are applied to reinterpret the classic formulation of the rule of law comprised of three elements which, when translated to the global level, are concerned with: developing non-arbitrary global governance; defining equality under IL; and the ordering of international legal power. The model is tested through a longitudinal case study of the International Criminal Court to demonstrate that American policy decision-making was structured by the theorised conceptions of IL, rather than by tactical compromises between “law” and “politics”. It is concluded that foreign policy ideology sets hard limits on the ability of the US to reach consensus on rule of IL ideals with even its closest allies.