The justices unanimously overturned the conviction of Carol Ann Bond of Lansdale, Pa. Bond, a microbiologist, smeared irritating chemicals on a car door, door knob and mailbox after she discovered that a close friend was pregnant by her husband.
Bond was found guilty of violating a 1998 law impementing the Chemical Weapons Convention.
Chief Justice John Roberts said the treaty and law "deals with crimes of deadly seriousness." But he said Bond hoped to cause "an uncomfortable rash."
"The global need to prevent chemical warfare does not require the federal government to reach into the kitchen cupboard, or to treat a local assault with a chemical irritant as the deployment of a chemical weapon," Roberts said.
Bond's lawyers challenged her conviction and six-year sentence to federal prison on the grounds that the prosecution violated the 10th Amendment, which reserves rights to the states not specifically granted the federal government. In 2011, the Supreme Court ordered a federal appeals court to consider the argument, but the Third Circuit U.S. Court of Appeals upheld the conviction.