Twenty-one years after the Rome Statute of the International Criminal Court was adopted, state noncompliance with politically-costly legal obligations remains the main barrier to international justice. This article explores two sequential questions: First, why do states ratify the Rome Statute? And second, how likely are their governments to genuinely comply with its provisions? This article employs an original methodology to explore these questions, combining a survey of diplomats posted to the UN Headquarters with thirty-seven structured interviews. Study results show that the (in)sincerity of state commitment reliably predicts future compliance with treaty obligations. Three main conclusions can be drawn from this study. First, it is crucial to distinguish between the moment of ratification and that of actual obligation. Second, the lawyerly assumption that state motives do not matter lacks empirical support. Third, the role of regional actors in enhancing compliance from state authorities deserves further recognition.