In Diaz v. United States, the U.S. Supreme Court ruled that a person can be tried and punished in both federal and territorial courts for the same act without violating the Double Jeopardy Clause of the Fifth Amendment to the Constitution. The case involved a man named Diaz who was charged with murder in both federal court (because he killed someone on an American naval vessel) and territorial court (because it happened within Philippine territory). He argued that being prosecuted twice constituted double jeopardy, but his argument was rejected by lower courts. On appeal, the Supreme Court affirmed these decisions, stating that because Congress had established separate sources of authority - one national and one local - over territories like Philippines at this time period, they could each enforce their own laws independently.

In the dissenting opinion for Diaz v. United States, Justice Harlan argued that the U.S. Constitution did not permit a person to be tried twice for the same offense in different jurisdictions (federal and state). He believed this violated the Double Jeopardy Clause of the Fifth Amendment, which states that no person shall "be subject for the same offence to be twice put in jeopardy of life or limb." Harlan contended that this clause should apply regardless of whether an act constituted an offense against both federal and state laws. He also expressed concern about potential abuses if multiple prosecutions were allowed, as it could lead to oppressive practices by government authorities seeking repeated trials until they secured a conviction. Thus, he disagreed with majority's ruling allowing Diaz's prosecution by federal authorities after his prior trial and acquittal on similar charges under Philippine law.