How are processes of political development structured across space and time by pre-existing institutions? This article develops a spatio-temporal theory of institutional change by analyzing the evolving infrastructural power of the European Union's (EU) legal order using geospatial methods. Specifically, while the enforcement of EU law in domestic courts has spread over time, both structural demand and the mobilizing efforts of institutional change agents cannot fully account for this process. Instead, we theorize that the pattern and pace of the domestic spread of EU law is shaped by pre-existing state institutions – particularly by the organization of domestic judiciaries. To assess this claim, we compare patterns of domestic judicial enforcement of EU law across France – a unitary state with a centralized judiciary – Italy – a weaker unitary state with a centralized judiciary – and Germany – a federal state with a decentralized judiciary. Promoting a geospatial approach to the study of legal integration and historical institutionalism more broadly, we use an original geocoded dataset of cases referred to the EU's European Court of Justice by national courts to unpack how the subnational reach of Europe’s supranational legal order remains conditioned by state institutions.