Writing for the majority in the 5-3 decision, Chief Justice John Roberts wrote that the law "falls well within the confines of the authority Congress chose to leave to the states," according to the Associated Press. Justices Stephen Breyer, Ruth Bader Ginsburg and Sonia Sotomayor dissented, while Justice Elena Kagan sat the case out because she had worked on it while serving as U.S. Solicitor General.
According to The New York Times, the decision "turned mostly on the meaning of a provision of a 1986 federal law, the Immigration Reform and Control Act, which said that it overrode 'any state or local law imposing civil or criminal sanctions (other than through licensing and similar laws) upon those who employ' unauthorized aliens." The majority of the justices found that states were entitled to supplement federal penalties with tougher local ones.
The law was signed back in 2007 by then-Governor Janet Napolitano, now Homeland Security Secretary. The challenge was brought by business interests and civil liberties groups, the AP reports.
A more recent Arizona law, SB 1070, permits law enforcement officials to demand immigration papers from anyone they suspected of being in the state illegally, has been the subject of much controversy. In April, a Ninth Circuit Court of Appeals panel upheld a lower court's decision to block key parts of that law until the Justice Department's lawsuit against it is decided.