Federal authorities were wrong to charge a Pennsylvania woman with violating an international chemical weapons ban after she tried to use poison against her former best friend in a dispute over her cheating husband, the US Supreme Court declared on Monday.

The high court ruled unanimously that prosecutors should not have charged Carol Anne Bond with violating a federal statute related to the Chemical Weapons Convention.

The unusual case involved a simple assault through the use of poison that resulted in a minor thumb burn, rather than an attempt to deploy toxic chemicals to cause mass casualties and engage in chemical warfare, the court said.

Use of the chemical weapons statute to prosecute Ms. Bond amounted to a substantial and unwarranted expansion of federal authority into an area of law adequately covered by state criminal statutes in Pennsylvania, the justices said.

“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,” Chief Justice John Roberts wrote in a 21-page decision.

He said there was no reason to suppose that Congress thought otherwise when it implemented the Chemical Weapons Convention.

All nine justices concurred in the judgment, but three conservative justices – Antonin Scalia, Clarence Thomas, and Samuel Alito – concluded that the federal statute clearly applied in the Bond case.

They concurred in the end result because, in their view, the underlying statute violated broader constitutional limits on congressional authority to impose such sweeping laws under a treaty. Their objections centered on the court using its own interpretation of the federal statute to avoid confronting the broader constitutional issue.

“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; and we have no authority to amend it,” Justice Scalia wrote in a concurrence.

Given that conclusion, Scalia said, the court had no alternative but to decide whether applying the chemical weapons statute to Bond’s conduct was constitutional. He concluded that the statute was unconstitutional.

The decision stems from a strange case involving an attempt by Bond, a microbiologist, to exact revenge against her former best friend, Myrlinda Haynes, after Bond discovered that her friend was pregnant and that the father of the child was Bond’s own husband.

The betrayal was all the more stinging because Bond had been unable to have children and had expressed her sincere joy for her girlfriend when she learned that Ms. Haynes was expecting a child.

Her joy turned to rage after learning the truth.

Rather than plan a baby shower, Bond stole a toxic chemical from her employer and purchased another ingredient. After mixing the chemicals into a poison concoction, Bond smeared the mixture on Haynes’s home doorknob, her car door handle, and her mailbox.

Haynes sustained a burn to her thumb but otherwise managed to avoid serious injury. She complained to local police, who refused to investigate.

Eventually, Haynes contacted postal inspectors, who have authority to investigate attempts to interfere with the delivery of mail. The inspectors set up surveillance cameras that identified Bond inserting chemicals into the muffler of Haynes’s car.

Federal agents arrested Bond and charged her with two counts of possessing and using a “chemical weapon.”

Bond’s lawyers argued that applying the chemical weapons law to a local poisoning case violated the 10th Amendment, which reserves general police powers to state and local governments.

A judge and a federal appeals court rejected Bond’s argument and upheld the government’s broad reading of the term “chemical weapon.”

In reversing the Third Circuit Court of Appeals, the high court said there was no clear indication that Congress meant for the chemical weapons statute to apply to such a broad range of conduct.

“No speaker in natural parlance would describe Bond’s feud-driven act of spreading irritating chemicals on Haynes’s door knob and mailbox as ‘combat,’ ” Chief Justice Roberts said.

He said the government’s broad definition of what amounted to a “chemical weapon” would “sweep in everything from the detergent under the kitchen sink to the stain remover in the laundry room.” He continued, “Yet no one would ordinarily describe those substances as ‘chemical weapons.’ ”

“Any parent would be guilty of a serious federal offense – possession of a chemical weapon – when, exasperated by the children’s repeated failure to clean the goldfish tank, he considers poisoning the fish with a few drops of vinegar,” Roberts said.

“We are reluctant to ignore the ordinary meaning of ‘chemical weapon’ when doing so would transform a statute passed to implement the international Convention on Chemical Weapons into one that also makes it a federal offense to poison goldfish.”

The case was Bond v. US (12-158).