The Supreme Court last week 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. Bond spread a caustic combination of easily obtainable chemicals on doorknobs, car handles and a mailbox. Myrlinda Haynes, the target, burned her thumb on tainted mail. Postal inspectors set up surveillance cameras and caught 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.

Federal officials should have left it to local authorities to pursue lesser, more appropriate charges against Bond. Instead, Bond served six years in federal prison.

Now, she is challenging her conviction by claiming that the government intruded into a state matter. Her advocates argue that the Constitution reserves policing powers for the states.

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

Both sides are right. 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.