Historical interest in popular constitutionalism has enlivened the search for the origins of judicial review. Several precursors of judicial review in the state courts during the 1780s, in particular, demand explanation. If early modern Anglo-Americans did not perceive courts as enforcers of constitutional limits on legislatures, what explains these attempts by judges to curtail legislation in the “critical period” before the Philadelphia Convention? This article argues that these cases involved antiloyalist legislation and related laws that violated the Peace Treaty of 1783 or the law of nations, or otherwise obstructed diplomatic and commercial relations with the other empires of the Atlantic world. Lawyers and judges drew on available legal scripts - such as the customary liberties of Englishmen and the notion of imperial supremacy - to argue that courts had the power to curb state legislation that infringed on these superior sources of law. This use of the courts fitted into a larger, Federalist constitutional program that was designed to reintegrate the United States into the Atlantic world.