Arizona’s Senate Bill 1070 has focused attention on whether federal law preempts the prosecution of state immigration crime in local criminal courts. Absent from the current discussion, however, is an appreciation of how Arizona’s existing body of criminal immigration law—passed well before SB 1070 and currently in force in the state—functions on the ground to regulate migration. Drawing on statistical data, prosecution policies, trial-level court records, and interviews with stakeholders, this Article is the first to investigate the practice of local immigration prosecution. It does so in the hotbed of immigration enforcement—Maricopa County, Arizona—through a detailed case study of the implementation of a 2005 Arizona alien smuggling law. Specifically, this Article reveals four key aspects of the national immigration system that have shifted in the face of state criminalization: the functional definition of immigration crime, the breadth of state immigration enforcement authority, the allocation of federal resources for criminal prosecution, and the exercise of executive control over immigration policy. Through this analysis, this Article shows how Arizona, despite the formal prohibition on state and local immigration regulation, has redefined and restructured the federal system for punishing immigration crime. In so doing, this Article fosters a richer and more accurate understanding of the role of the local prosecutor in immigration federalism.