Policing is a prominent but understudied part of American politics. This article asks: Why did some, but not all, American states adopt a state police force in the early twentieth century? The state police force—a statewide policing agency with general jurisdiction over crimes throughout a state—was a prized progressive policy reform for decades. Yet many states declined to adopt the innovation. That puzzle becomes even more interesting given that all forty-eight states adopted the closely related innovation of a state highway patrol during the same period. This article applies diffusion theory to explain the origins of a familiar feature of American policing. Using a multimethod research design, I found that labor-capital struggles and regional pressures for diffusion were most important in shaping the adoption of state police forces. By contrast, adoptions of highway patrols appear to have been influenced by factors such as urbanization and fiscal capacity.