THE SUPREME COURT on Tuesday considered a case involving two family disputes. One was a love triangle that turned dangerous. The other was the perpetual push-and-pull between the states and the federal government to define their proper roles. The justices must make sure they do not aggravate the latter in addressing the former.

In 2006 and 2007, Carol Anne Bond tried to poison the mother of her husband’s child. Ms. Bond spread a caustic orange combination of easily obtainable chemicals on door knobs, car handles and a mailbox. Myrlinda Haynes, the target, burned her thumb on tainted mail. After several complains, postal inspectors set up surveillance cameras and caught Ms. Bond in the act.

Federal authorities tried her for violating a law Congress passed to bring the United States into compliance with the Chemical Weapons Convention, a treaty meant to eliminate the use of weapons of mass destruction.

This ridiculous overcharge, so obviously incongruous with the aims of the treaty, should have never happened. Federal officials should have left it to local authorities to pursue lesser, more appropriate charges against Ms. Bond. Instead, Ms. Bond served six years in federal prison.

Now, she is challenging her conviction by claiming that the government intruded into a state matter. Ms. Bond’s advocates argue that the constitution reserves policing powers for the states, even when the feds are attempting to enforce a legitimate international treaty.

In response, the federal government made a strong case that it must have authority to enforce treaties. Otherwise, its ability to commit the country to valid agreements with other nations on matters of international concern would degrade.

Both sides are right. Ms. Bond’s crime had nothing to do with chemical weapons as normally understood, and the feds shouldn’t have charged her as if it did. But the federal government does have an obligation to make sure treaties are enforced. We hope the justices find a way to rebuke the overreach without undermining the authority of the president and Congress to negotiate and apply treaties.