Printz v. United States, 521 U.S. 898 (1997), held that the federal government cannot force state or local officials to enforce federal law. The case struck down a provision requiring local sheriffs to conduct background checks for gun purchases.
The Court ruled this violated state sovereignty by "commandeering" state executive officials. This anti-commandeering doctrine now protects sanctuary cities that refuse to help ICE with deportations—states can decline federal enforcement requests without facing penalties.