Over the past two decades, sub-federal participation has become a significant feature of the immigration enforcement landscape. Much of this participation is not sanctioned by the letter of federal immigration law, and recently, the federal government’s immigration enforcement policies have moved in a direction aimed at bringing sub-federal enforcement efforts more closely into alignment with the letter of that law. The Court’s decision in Arizona v. United States is insufficiently attentive both to the letter of federal law and to ongoing federal practice. And notably, in Arizona, as in Chamber of Commerce v. Whiting, the Court discounts the antidiscrimination goals of federal immigration law. This article elaborates upon these arguments, and concludes that, in the wake of Arizona v. United States, state and local law enforcement will continue to play a leading role in shaping immigration enforcement and the immigrant experience in the United States, notwithstanding the Court’s formal endorsement of federal primacy in this realm. Part I of this Article outlines the Court’s immigration federalism jurisprudence, focusing on its recent decisions. Part II explores the reasons that the Court’s formal adherence to traditional notions of immigration federalism will fail to translate into federal primacy in practice. Succinctly put, traditional judicial articulations of immigration federalism do not account for the sub-federal immigration enforcement discretion that has accumulated over the past two decades. Part III of this Article unpacks the Court’s decision in United States v. Arizona to explain why the seemingly traditional approach to federalism espoused by the Court actually represents a substantial reformulation of immigration federalism principles on the ground.