WASHINGTON — A unanimous Supreme Court ruled Monday that prosecutors may not rely on an international chemical weapons treaty to convict a woman who attacked her husband's mistress.

The justices threw out the conviction of Carol Anne Bond of Lansdale, Pa., who was prosecuted under a 1999 law based on the chemical weapons treaty. Bond served a six-year prison term after being convicted of using toxic chemicals that caused a thumb burn on a friend who had become her husband's lover.

The intent of the chemical weapons treaty was to prevent a repeat of the use of mustard gas in World War I or toxic weapons in the Iraq-Iran war in the early 1980s, not "an amateur attempt by a jilted wife to injure her husband's lover," Chief Justice John Roberts wrote for the court.

Roberts contrasted John Singer Sargent's massive painting, "Gassed," with its depictions of men who have been blinded by mustard gas, with Bond's actions. "There are no life-sized paintings of Bond's rival washing her thumb," he said.

Pennsylvania laws are sufficient to deal with threats posed by a woman in a love triangle, he said.

"In sum, 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.

The case posed potentially significant questions about the federal government's power to make and enforce treaties. The justices resolved the case without reaching that issue, although Justices Samuel Alito, Antonin Scalia and Clarence Thomas said they would have.

Bond, unable to bear any children of her own, was excited for her best friend Myrlina Haynes when the woman announced her pregnancy. But that was before Bond learned that the baby's father was her husband of more than 14 years, Clifford Bond.

Vowing revenge, Bond, a laboratory technician, stole the chemical 10-chloro-10H phenoxarsine from the company where she worked and purchased potassium dichromate on Amazon.com. Both can be deadly if ingested or exposed to the skin at sufficiently high levels.

Bond's efforts were obvious enough that Haynes noticed chemicals had been spread on her door handle and in the tailpipe of her car. Haynes suffered a minor burn. But believing local police did not do enough to investigate, she called the United States Postal Service after finding more of the chemicals on her mailbox. Postal inspectors arrested Bond after they videotaped her going back and forth between Haynes' car and the mailbox with the chemicals.

Instead of turning the domestic dispute case over to state prosecutors, a federal grand jury indicted Bond on two counts of possessing and using a chemical weapon. The grand jury based the charges on a federal anti-terrorism law passed to fulfill the United States' international treaty obligations under the 1993 Convention on the Prohibition of the Development, Production, Stockpiling and use of Chemical Weapons and on Their Destruction.

Bond pleaded guilty.

In an earlier stage of the case, the high court unanimously rejected prosecutors' arguments and lower court rulings that prevented Bond from even challenging her conviction.

The case is Bond v. U.S., 12-158.