Chief Justice Roberts' majority opinion in Bond v. United States has been sharply criticized (see here and here), so I'll say few words partially in its favor. The case has seemed odd from the beginning because the federal statute at issue implements the Chemical Weapons Convention and (as the majority says) the local misuse of household chemicals does not seem the type of activity that is likely to implicate an international treaty. As the majority puts it:
To begin, as a matter of natural meaning, an educated user of English would not describe Bond's crime as involving a "chemical weapon." Saying that a person "used a chemical weapon" conveys a very different idea than saying the person "used a chemical in a way that caused some harm." The natural meaning of "chemical weapon" takes account of both the particular chemicals that the defendant used and the circumstances in which she used them.
When used in the manner here, the chemicals in this case are not of the sort that an ordinary person would associate with instruments of chemical warfare. The substances that Bond used bear little resemblance to the deadly toxins that are "of particular danger to the objectives of the Convention." Why We Need a Chemical Weapons Convention and an OPCW, in Kenyon & Feakes 17 (describing the Convention's Annex on Chemicals, a nonexhaustive list of covered substances that are subject to special regulation). More to the point, the use of something as a "weapon" typically connotes "[a]n instrument of offensive or defensive combat," Webster's Third New International Dictionary 2589 (2002), or "[a]n instrument of attack or defense in combat, as a gun, missile, or sword," American Heritage Dictionary 2022 (3d ed. 1992). But 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." Nor do the other circumstances of Bond's offense-an act of revenge born of romantic jealousy, meant to cause discomfort, that produced nothing more than a minor thumb burn-suggest that a chemical weapon was deployed in Norristown, Pennsylvania. Potassium dichromate and 10-chloro-10H-phenoxarsine might be chemical weapons if used, say, to poison a city's water supply. But Bond's crime is worlds apart from such hypotheticals, and covering it would give the statute a reach exceeding the ordinary meaning of the words Congress wrote.
It's true, as Justice Scalia says in concurrence, that the statute defines "chemical weapon" in a way that appears to vary sharply from the phrase's ordinary meaning. But according to the majority, it is appropriate to consider "the dissonance between [the] ordinary meaning and the reach of the definition." As the majority says,
...[W]e have doubts that a treaty about chemical weapons has anything to do with Bond's conduct. The Convention, a product of years of worldwide study, analysis, and multinational negotiation, arose in response to war crimes and acts of terrorism. See Kenyon & Feakes 6. There is no reason to think the sovereign nations that ratified the Convention were interested in anything like Bond's common law assault.
That seems enough to create an ambiguity, or at least an oddity, such that the majority's invocation of a presumption against Congress upsetting the state/federal balance may be appropriate.
Where I think the majority goes astray, though, is in its focus on the statute rather than the Convention. The majority says:
Fortunately, we have no need to interpret the scope of the Convention in this case. Bond was prosecuted under section 229 [of the implementing statute], and the statute--unlike the Convention--must be read consistent with principles of federalism inherent in our constitutional structure.
That seems wrong on two counts. First, the statute's language in relevant part closely tracks the Convention's language. Surely what Congress wanted to do was to fully implement the Convention. To ask what Congress intended is to ask what the Convention intended. If the Convention reached very broadly into local matters, Congress would want to do so as well. There is no reason to think -- given the parallel language -- that Congress wanted to do less than the Treaty called for. And indeed the majority seems to see this, for despite its disclaimer, it repeatedly returns to the purpose and context of the Convention.
Second, I think it's wrong to say that the Convention, unlike the statute, need not "be read consistent with principles of federalism." The U.S. Senate gave its consent to the Convention. If Congress is presumed not to upset the state/federal balance by legislation absent a clear statement, why should this presumption not also extend to the Senate (a part of Congress) when it approves treaties? Further, treaties should be subject to a related presumption that they do not reach matters of purely local concern. As Justice Thomas says in his concurrence, a treaty is by definition an agreement on matters of international concern. That category may be somewhat hard to define (especially in the modern context in which some matters occurring entirely within a sovereign nation nonetheless attract intense international interest). But however defined, it seems plain that Bond's actions were not of international concern. I cannot imagine that any nation would show any interest in how the U.S. treated her offense.
In sum, I think the proper focus is the Convention, not the statute. The statute was designed to cover whatever the Convention covered. But the ambiguity the majority identifies in the statute is a product of a parallel ambiguity in the Convention, which also appears to define "chemical weapons" broadly and in a counterintuitive way. The Convention, if read as the government would read it, would reach matters not of international concern and would upset the state/federal balance with respect to local crime. In this context, it is plausible to say that is not what the Senate understood itself to be approving.
(For my earlier assessment of the case, somewhat along these lines, see here).
UPDATE: At Re's Judicata, Richard Re has an interesting take on the majority opinion's implications for federalism: Bond and the Doctrine of One Last Chance.