The decision was a major legal and political victory for President Obama.
The justices did temporarily uphold one provision, one that allows police officers in Arizona to check the immigration status of those they arrest. But support for the provision was highly conditional.
"The United States has established that [the stricken provisions of the state law] are pre-empted," Justice Anthony Kennedy said in the prevailing opinion. "It was improper, however, [for a federal judge] to enjoin [the upheld provision] before the state courts had an opportunity to construe it and without some showing that enforcement of the provision in fact conflicts with federal immigration law and its objectives."
All nine justices joined Kennedy in the main parts of his opinion. Three issued partial dissents, with Justice Antonin Scalia saying he would uphold the entire law. Justice Elena Kagan, who was U.S. solicitor general in the earlier stages of the case, did not take part in the Supreme Court case.
Kennedy said the federal government's "broad, undoubted power over immigration and alien status rests, in part, on its constitutional power to 'establish an uniform rule of Naturalization' ... and on its inherent sovereign power to control and conduct foreign relations.
He also noted the Constitution's supremacy clause -- federal law trumps state law -- gives Congress the power to pre-empt state law.
Section 3 ... makes "failure to comply with federal alien-registration requirements a state misdemeanor; section 5(C) makes it a misdemeanor for an unauthorized alien to seek or engage in work in the state; section 6 authorizes state and local 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'; and section 2(B) requires officers conducting a stop, detention, or arrest to make efforts, in some circumstances, to verify the person's immigration status with the federal government."
The opinion said, "It is not clear at this stage and on this record that section 2(B), in practice, will require state officers to delay the release of detainees for no reason other than to verify their immigration status. This would raise constitutional concerns. And it would disrupt the federal framework to put state officers in the position of holding aliens in custody for possible unlawful presence without federal direction and supervision. But section 2(B) could be read to avoid these concerns. If the law only requires state officers to conduct a status check during the course of an authorized, lawful detention or after a detainee has been released, the provision would likely survive pre-emption -- at least absent some showing that it has other consequences that are adverse to federal law and its objectives. Without the benefit of a definitive interpretation from the state courts, it would be inappropriate to assume section 2(B) will be construed in a way that conflicts with federal law."
The opinion affirms in part and reverses in part a ruling by a federal appeals court.