From the majority opinion (the Court was unanimous on the result, but not the reasoning):

[I]t is appropriate to refer to basic principles of federalism embodied in the Constitution to resolve ambiguity in a federal statute. In this case, the ambiguity derives from the improbably broad reach of the key statutory definition given the term — “chemical weapon” — being defined; the deeply serious consequences of adopting such a boundless reading; and the lack of any apparent need to do so in light of the context from which the statute arose — a treaty about chemical warfare and terrorism. We conclude that, in this curious case, we can insist on a clear indication that Congress meant to reach purely local crimes, before interpreting the statute’s expansive language in a way that intrudes on the police power of the States….
In settling on a fair reading of a statute, it is not unusual to consider the ordinary meaning of a defined term, particularly when there is dissonance between that ordinary meaning and the reach of the definition….
The Government’s reading of section 229 would “ ‘alter sensitive federal-state relationships,’ ” convert an astonishing amount of “traditionally local criminal conduct” into “a matter for federal enforcement,” and “involve a substantial extension of federal police resources.” It would transform the statute from one whose core concerns are acts of war, assassination, and terrorism into a massive federal anti-poisoning regime that reaches the simplest of assaults. As the Government reads section 229, “hardly” a poisoning “in the land would fall outside the federal statute’s domain.” Of course Bond’s conduct is serious and unacceptable — and against the laws of Pennsylvania. But the background principle that Congress does not normally intrude upon the police power of the States is critically important. In light of that principle, we are reluctant to conclude that Congress meant to punish Bond’s crime with a federal prosecution for a chemical weapons attack….

The facts:

Petitioner Carol Anne Bond is a microbiologist from Lansdale, Pennsylvania. In 2006, Bond’s closest friend, Myrlinda Haynes, announced that she was pregnant. When Bond discovered that her husband was the child’s father, she sought revenge against Haynes. Bond stole a quantity of 10-chloro-10H-phenoxarsine (an arsenic-based compound) from her employer, a chemical manufacturer. She also ordered a vial of potassium dichromate (a chemical commonly used in printing photographs or cleaning laboratory equipment) on Both chemicals are toxic to humans and, in high enough doses, potentially lethal. It is undisputed, however, that Bond did not intend to kill Haynes. She instead hoped that Haynes would touch the chemicals and develop an uncomfortable rash.
Between November 2006 and June 2007, Bond went to Haynes’s home on at least 24 occasions and spread the chemicals on her car door, mailbox, and door knob. These attempted assaults were almost entirely unsuccessful. The chemicals that Bond used are easy to see, and Haynes was able to avoid them all but once. On that occasion, Haynes suffered a minor chemical burn on her thumb, which she treated by rinsing with water. Haynes repeatedly called the local police to report the suspicious substances, but they took no action. When Haynes found powder on her mailbox, she called the police again, who told her to call the post office. Haynes did so, and postal inspectors placed surveillance cameras around her home. The cameras caught Bond opening Haynes’s mailbox, stealing an envelope, and stuffing potassium dichromate inside the muffler of Haynes’s car.
Federal prosecutors naturally charged Bond with two counts of mail theft, in violation of 18 U. S. C. §1708. More surprising, they also charged her with two counts of possessing and using a chemical weapon, in violation of section 229(a). Bond moved to dismiss the chemical weapon counts on the ground that section 229 exceeded Congress’s enumerated powers and invaded powers reserved to the States by the Tenth Amendment. The District Court denied Bond’s motion. She then entered a conditional guilty plea that reserved her right to appeal. The District Court sentenced Bond to six years in federal prison plus five years of supervised release ….

Section 229(a) was enacted by Congress to implement the Convention on the Prohibition of the Development, Production, Stockpiling, and Use of Chemical Weapons and on Their Destruction, an international treaty; the statute follows the treaty in defining “chemical weapon”:

It defines “chemical weapon” in relevant part as “[a] toxic chemical and its precursors, except where intended for a purpose not prohibited under this chapter as long as the type and quantity is consistent with such a purpose.” “Toxic chemical,” in turn, is defined in general as “any chemical which through its chemical action on life processes can cause death, temporary incapacitation or permanent harm to humans or animals. The term includes all such chemicals, regardless of their origin or of their method of production, and regardless of whether they are produced in facilities, in munitions or elsewhere.” Finally, “purposes not prohibited by this chapter” is defined as “[a]ny peaceful purpose related to an industrial, agricultural, research, medical, or pharmaceutical activity or other activity,” and other specific purposes.

Justices Scalia, Thomas, and Alito each wrote opinions concurring in the judgment, reaching the result for a different reason. They concluded the statute clearly did cover Bond’s conduct. (Justices Scalia and Thomas joined each other’s opinions fully, and Justice Alito joined portions of Justice Scalia’s and Thomas’s opinions.) Among other things, they concluded that the majority’s limiting construction was both inconsistent with the text of the statute, and itself unclear in scope (some paragraph breaks added):

A criminal statute must clearly define the conduct it proscribes. If it does not “ ‘give a person of ordinary intelligence fair notice’ ” of its scope, United States v. Batchelder, 442 U. S. 114, 123 (1979), it denies due process.
The new §229(a)(1) fails that test. Henceforward, a person “shall be fined …, imprisoned for any term of years, or both,” §229A(a)(1) — or, if he kills someone, “shall be punished by death or imprisoned for life,” §229A(a)(2) — whenever he “develop[s], produce[s], otherwise acquire[s], transfer[s] directly or indirectly, receive[s], stockpile[s], retain[s], own[s], possess[es], or use[s], or threaten[s] to use,” §229(a)(1), any chemical “of the sort that an ordinary person would associate with instruments of chemical warfare” (emphasis added).
Whether that test is satisfied, the Court unhelpfully (and also illogically) explains, depends not only on the “particular chemicals that the defendant used” but also on “the circumstances in which she used them.” The “detergent under the kitchen sink” and “the stain remover in the laundry room” are apparently out — but what if they are deployed to poison a neighborhood water fountain? Poisoning a goldfish tank is also apparently out, [see majority opinion] at 17, but what if the fish belongs to a Congressman or Governor and the act is meant as a menacing message, a small-time equivalent of leaving a severed horse head in the bed? See ibid. (using the “concerns” driving the Convention — “acts of war, assassination, and terrorism” — as guideposts of statutory meaning).
Moreover, the Court’s illogical embellishment seems to apply only to the “use” of a chemical, but “use” is only 1 of 11 kinds of activity that the statute prohibits. What, one wonders, makes something a “chemical weapon” when it is merely “stockpile[d]” or “possess[ed]?” To these questions and countless others, one guess is as bad as another.
No one should have to ponder the totality of the circumstances in order to determine whether his conduct is a felony. Yet that is what the Court will now require of all future handlers of harmful toxins — that is to say, all of us. Thanks to the Court’s revisions, the Act, which before was merely broad, is now broad and unintelligible. “[N]o standard of conduct is specified at all.” Before long, I suspect, courts will be required to say so.

But the concurring Justices concluded that the statute exceeded Congress’s power, because of their conclusions that the Treaty Clause doesn’t empower Congress to regulate domestic behavior that would be outside Congress’s power absent a treaty — a matter that has been hotly contested. (In the process, Justice Scalia’s concurrence, joined by Justice Thomas, cited our own Nick Rosenkranz’s “Executing the Treaty Power,” 118 Harv. L. Rev. 1867 (2005), and Will Baude’s “Rethinking the Federal Eminent Domain Power,” 122 Yale L. J. 1738 (2013).) The other Justices didn’t directly express an opinion on this constitutional question, because their interpretation of the statute made such an opinion unnecessary in their view.