Why has it been so difficult to reform U.S. policing? We provide a theoretical argument that understanding of the entrenched militarisation and accountability problems of U.S. police departments would benefit from using theory in comparative research on civil–military relations. American police forces undermine local democracy by encroaching upon the decision-making powers of city officials in ways that resemble militaries in fragile democracies. Applying historical and contemporary evidence and existing scholarly research on policing, we explain police militarisation was initiated by civilian leaders of city governments to garner governmental legitimacy, and by-proxy police support, in racialised contexts. Trading off city governments’ institutional strength in order to maintain legitimacy produced opportunities for police insubordination or subversion of city government oversight of police activity. Consequently, cities with low public legitimacy and/or weak municipal institutions, faced with high demands by militarised police departments, may be more likely to experience police subversion of democratic accountability over police activity.