A love triangle that ended with a woman poisoning her pregnant rival spawned a debate over chemical weapons, international relations, federalism and chocolate at the Supreme Court on Tuesday, with justices left trying to make sense of how a jealous wife ended up being prosecuted for violating an international chemical weapons treaty.

Carol Anne Bond of Lansdale, Pa., is challenging her conviction, saying the federal government’s decision to charge her using a chemical weapons law was an unconstitutional reach into a state’s power to handle what her lawyer calls a domestic dispute.

Bond, unable to have children of her own, was excited when her best friend Myrlina Haynes announced her pregnancy. But Bond found out her husband of more than 14 years was the father.

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 combined and spread the chemicals on Haynes’ door handle and in the tailpipe of Haynes’ car. Haynes noticed the bright orange compound but could not get local police interested in investigating.

When Haynes found the compound on her mailbox, she complained again to police, who told her to call the U.S. Postal Service. Federal investigators videotaped Bond going back and forth between Haynes’ car and the mailbox with the chemicals. Postal inspectors arrested Bond.

“The state of Pennsylvania exercised its prosecutorial discretion not to pursue this matter,” Bond’s lawyer said.

But a federal grand jury indicted Bond on two counts of possessing and using a chemical weapon, applying a federal anti-terrorism law. The law was passed to fulfill 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 and was given six years in prison. This is her second time in front of the Supreme Court. In 2011, the court unanimously sided with Bond to allow her to challenge her conviction. Lower courts subsequently rejected the appeal, leading to her current challenge.

A couple of justices were very critical of government prosecutors for choosing even to prosecute Bond using the chemical weapons law. “If you told ordinary people that you were going to prosecute Ms. Bond for using a chemical weapon, they would be flabbergasted,” said Justice Samuel Alito. “It’s so far outside of the ordinary meaning of the word.”

Justice Anthony Kennedy said it “seems unimaginable that you would bring this prosecution.”

The chemical weapons law makes it illegal to “develop, produce, otherwise acquire, transfer directly or indirectly, receive, stockpile, retain, own, possess or use, or threaten to use, any chemical weapon.”

Justices went down a long list of everyday items that could be prosecuted under the law, including the use of kerosene, matches, performance-enhancing drugs used in sports, and even vinegar — which would poison goldfish if put in a fishbowl.

“Would it shock you if I told you that a few days ago my wife and I distributed toxic chemicals to a great number of children?” Alito said to laughter from the courtroom. “On Halloween, we gave them chocolate bars. Chocolate is poison to dogs, so it’s a toxic chemical under the chemical weapons” law.

But Justices Ruth Bader Ginsburg and Elena Kagan pointed out that since the treaty was legal, then the implementing legislation would have to be legal as well, a position Solicitor General Donald Verrilli supported.