Arizona has been literally invaded by foreign nationals from across the Mexican border. With the federal government doing nothing to stem the tide of illegal aliens, Arizona passed its own laws to do so. Arizona bears many of the consequences of unlawful immigration. Hundreds of thousands of deportable aliens are apprehended in Arizona each year. Unauthorized aliens who remain in the State comprise almost six percent of the population. Accounts in the record suggest there is an 'epidemic of crime, safety risks, serious property damage, and environmental problems' associated with the influx of illegal migration across private land near the Mexican border. Section 3 of the law makes failure to comply with federal alien-registration requirements a state misdemeanor. Section 5 makes it a misdemeanor for an unauthorized alien to seek or engage in work in the State. Two other provisions give specific arrest authority and investigative duties with respect to certain aliens to state and local law enforcement officers. Section 6 authorizes officers to arrest without a warrant a person 'the officer has probable cause to believe ... has committed any public offense that makes the person removable from the United States.' Section 2(B) provides that officers who conduct a stop, detention, or arrest must in some circumstances make efforts to verify the person's immigration status with the Federal Government. The District Court issued a preliminary injunction preventing the four provisions at issue from taking effect. The Ninth Circuit affirmed. The Supreme Court granted certiorari.

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