Arizona enacted SB 1070 to address unauthorized immigration, and the United States sought to enjoin four provisions as preempted by federal law. The Supreme Court held that three provisions could not take effect: section 3, which created a state misdemeanor for failing to comply with federal alien-registration requirements; section 5(C), which criminalized unauthorized work by an alien; and section 6, which authorized state officers to arrest a person believed to have committed a removable offense. Federal law occupied the field of alien registration, made the state employment penalty an obstacle to the federal employment framework, and limited state participation in immigration enforcement in ways inconsistent with section 6. The Court declined to enjoin section 2(B), which required officers conducting certain stops or arrests to check immigration status. Before state courts construed that provision, the record did not establish that it would conflict with federal law; it could be applied during a lawful detention or after release without creating the feared conflict. The judgment was therefore affirmed in part, reversed in part, and remanded. Justices Scalia, Thomas, and Alito concurred in part and dissented in part, while Justice Kagan took no part.