In Reina v. United States, the Supreme Court was tasked with deciding whether a federal court could compel testimony from a witness who had been granted immunity in state proceedings but not in federal ones. The case arose when an individual named Reina refused to testify before a grand jury about matters for which he had received immunity at the state level, arguing that this protection should extend to his federal testimony as well. However, the Supreme Court disagreed and held that separate sovereigns (i.e., states and the federal government) can independently grant or withhold immunity; therefore, unless explicitly stated otherwise by Congress, protections offered at one level of government do not automatically apply at another. This ruling clarified how testimonial immunities function across different jurisdictions within America's dual-sovereign system.

In the dissenting opinion for Reina v. United States, Justice Brennan disagreed with the majority's decision to allow federal courts to compel testimony from a grand jury witness who had already testified before a state grand jury on the same matter. He argued that this violated principles of federalism and comity between states and federal government. He contended that compelling such testimony could potentially undermine state criminal prosecutions by exposing their evidence prematurely or discouraging witnesses from testifying in state proceedings out of fear they may be compelled to testify again at the federal level. Furthermore, he expressed concern about potential abuse of power by prosecutors using multiple grand juries as an intimidation tactic against witnesses. Therefore, he believed it was necessary for there to be significant justification before allowing such compulsion which he did not see present in this case.