Puerto Rico sought extradition of a man charged with vehicular homicide; Iowa's governor refused. The Court unanimously overruled the Civil War-era Kentucky v. Dennison (1861), which had held governors' extradition duty judicially unenforceable. Justice Marshall wrote that Dennison rested on 'a fundamental premise since rejected' — that federal courts lack authority to command state executive action to enforce the Constitution. Under Article IV, § 2, extradition is a mandatory federal duty and the federal courts may issue mandamus to enforce it. Branstad closed a 126-year gap in the machinery of interstate criminal justice.