Arizona v. United States, 567 U.S. 387 (2012), struck down most of Arizona's SB 1070 immigration law, holding that federal law preempts state immigration enforcement. The Court found that immigration policy is an area of exclusive federal authority—states cannot create their own immigration enforcement systems. However, the Court upheld the "show me your papers" provision allowing police to check immigration status during lawful stops. This creates tension: states can't commandeer federal resources, but the federal government can't commandeer state police.