The U.S. Supreme Court ruled unanimously today against the Obama administration in a major case testing the reach of federal power.

At issue in Bond v. United States was the conviction of Carol Anne Bond, a Pennsylvania woman sentenced to six years in federal prison under the Chemical Weapons Implementation Act after she smeared two toxic substances on the door knob and car door of a woman who had been carrying on an affair with Bond's husband. According to Bond, the federal government exceeded its enumerated powers by making a federal crime out of her purely local offense. Today, the Supreme Court ruled in Bond's favor.

The Obama administration's "boundless" interpretation of the chemical weapons law, declared the opinion of Chief Justice John Roberts, "would transform the statute from one whose core concerns are acts of war, assassination, and terrorism into a massive federal anti-poisoning regime that reaches the simplest of assaults."

Joined by Justices Anthony Kennedy, Ruth Bader Ginsburg, Sonia Sotomayor, and Elena Kagan, Roberts found that the federal law simply had no application to "an amateur attempt by a jilted wife to injure her husband's lover, which ended up causing only a minor thumb burn readily treated by rinsing with water." The power to prosecute such acts rests entirely in the hands of the states, the Court concluded. "There is no reason to think the sovereign nations that ratified the [Chemical Weapons] Convention were interested in anything like Bond's common law assault."

Writing separately, Justices Antonin Scalia, Clarence Thomas, and Samuel Alito concurred in the view that Bond's conviction should be overturned, but argued that Roberts' narrow ruling did not go far enough. In contrast to Roberts, these three justices argued that the chemical weapons law did cover Bond's conduct, and therefore the law should be struck down on constitutional grounds. "As sweeping and unsettling as the Chemical Weapons Convention Implementation Act of 1998 may be, it is clear beyond doubt that it covers what Bond did," wrote Justice Scalia. "So we are forced to decide—there is no way around it—whether the Act's application to what Bond did was constitutional. I would hold that it was not."

The opinion in Bond. v. United States is available here.