567 U.S. 387 (2012)
Arizona v. United States is a modern cornerstone of federalism and preemption doctrine in the immigration context.
Whether federal immigration law, under the Supremacy Clause, preempts Arizona S.B. 1070's provisions Sections 2(B), 3, 5(C), and 6, which create and expand state-level immigration enforcement mechanisms.
Under the Supremacy Clause, state laws are preempted when Congress occupies a field (field preemption), when state law conflicts with federal law (conflict preemption), or when state law stands as an obstacle to the accomplishment and execution of federal objectives (obstacle preemption). The federal government has broad, predominant authority over immigration, including alien registration (Hines v. Davidowitz), admission, removal, and the regulation of unauthorized employment (IRCA). States may cooperate with federal authorities, including by sharing immigration status information and participating through federally authorized programs (e.g., 8 U.S.C. §§ 1357(g), 1373(c)), but may not create independent crimes or enforcement regimes that intrude upon or frustrate Congress's comprehensive framework and federal enforcement discretion.
The Court held that Sections 3 (state alien registration crime), 5(C) (criminalization of unauthorized workers seeking or engaging in work), and 6 (state warrantless arrest authority based on removability) are preempted by federal law. Section 2(B) (status-check provision) is not preempted on its face, but its implementation remains subject to constitutional constraints and as-applied challenges.
Arizona v. United States is a leading preemption case delineating the boundary between permissible state cooperation and impermissible state policymaking in immigration enforcement. It crystallizes the difference between field preemption (alien registration) and obstacle preemption (worker sanctions, arrest authority), and it elevates the importance of federal discretion in removal priorities. For law students, the case is essential for mastering the Supremacy Clause, the structure of the INA and IRCA, and how courts evaluate facial challenges versus as-applied challenges in a sensitive area touching foreign affairs, civil rights, and federal executive power.