The Supreme Court hears its most consequential case so far this term on Tuesday, and the question boils down to this: Can the federal government use a treaty with foreign nations to vitiate police powers that the Constitution reserves for the 50 states?

That may seem esoteric, but the question goes to the heart of the Constitution's separation of powers. That doctrine limits the writ of Congress by reserving certain powers to the states and the people. The last century has seen Washington wrest ever more power from the states, but even in his misguided ObamaCare opinion last year Chief Justice John Robertsrecognized limits on the reach of the Commerce Clause. Here's another opportunity, this time concerning the treaty power.

In Bond v. U.S., Carol Bond of Lansdale, Pennsylvania concedes that she spread a toxic chemical she had obtained from her employer on the car and mailbox of a friend who had an affair with her husband. The chemical did minimal harm despite her clear intent to do so. But federal prosecutors intervened in what would normally be a state criminal case to charge Mrs. Bond with violating the chemical-weapons convention that the Senate ratified in 1997.

Yes, you read that right. Mrs. Bond was convicted of waging chemical warfare—unlike Syria's Bashar Assad, who has been charged with nothing. The treaty may not get its man overseas, but it did get Mrs. Bond.

She is now challenging her conviction on grounds that the chemical treaty's 1998 implementing legislation cannot rewrite the U.S. Constitution. The case offers the Justices a chance to uphold federalism against Washington's misuse of the treaty power. If prosecutors can charge Mrs. Bond with violating a chemical-warfare treaty, then they can use any treaty to usurp state powers. For example, why not use Nafta's rules on trucks to prosecute highway violations?

In a previous turn at the High Court two years ago, the Justices ruled 9-0 that Mrs. Bond could challenge her conviction under the Tenth Amendment that reserves unspecified powers to the states and people. But on a second pass the lower courts still upheld her chemical conviction. That shows the need for the Justices to establish a clear limiting principle on treaty interpretation so federal officials cannot usurp state police powers.

Some liberals claim this is no big deal because the U.S. now ratifies few treaties, but no one knows if this will continue. The Founders required two-thirds of the Senate to ratify treaties because they envisioned relatively few such pacts that would involve large issues. But modern treaties are ever-more detailed and often require more extensive implementation language. Without limits from the Supreme Court, the treaty power is a dagger aimed at the heart of federalism.

Liberals and the Third Circuit Court of Appeals also point to a single sentence in a 1920 opinion by Oliver Wendell Holmes(Missouri v. Holland) that whatever Congress does to implement a valid treaty is legal under the Constitution's Necessary and Proper Clause. But one sentence is hardly definitive as a precedent, and in any case Holmes wrote before Congress went on its 90-year rampage against the separation of powers.

Today's Justices know better that Congress needs constitutional guardrails or it will drive the states off the road. Bondis a made-to-order opening for the High Court, and especially for Justice Anthony Kennedy, to reassert federalism as the Constitution's bulwark of liberty.