Arizona v. United States

Arizona's S.B. 1070 (defendant) created state penalties for failing to carry alien registration documents, for unauthorized aliens seeking work, and authorized warrantless arrests based on probable cause of removability, while a separate provision required officers to check immigration status during lawful stops; the federal government (plaintiff) sought to enjoin all four provisions as preempted, and the lower courts enjoined them all.

Whether a state law that addresses immigration and alien registration is preempted where Congress has completely occupied the entire field.