164
Opinion of the Court
reach. Nor does the use by this Court of the words "precise acts" in the leading case in which this Court has applied the Act, Williams, 327 U. S., at 717, help the Government in this respect. In Williams, the Court held that the ACA did not assimilate a State's "statutory rape" crime (with a cut-off age of 18) both because federal adultery and fornication statutes covered the defendant's "precise acts," and because the policies underlying a similar federal statute (with a cutoff age of 16) made clear there was no gap to fill. Id., at 724-725. The Court's opinion refers to both of these circumstances and does not decide whether the Act would, or would not, have applied in the absence of only one. We cannot find a convincing justification in language, purpose, or precedent for the Government's interpretation. Hence, we conclude that, just as a literal interpretation would produce an ACA that is too narrow, see supra, at 161-162, so the Government's interpretation would produce an ACA that is too broad.
In our view, the ACA's language and its gap-filling purpose taken together indicate that a court must first ask the question that the ACA's language requires: Is the defend-ant's "act or omission . . . made punishable by any enactment of Congress." 18 U. S. C. § 13(a) (emphasis added). If the answer to this question is "no," that will normally end the matter. The ACA presumably would assimilate the statute. If the answer to the question is "yes," however, the court must ask the further question whether the federal statutes that apply to the "act or omission" preclude application of the state law in question, say, because its application would interfere with the achievement of a federal policy, see Johnson v. Yellow Cab Transit Co., 321 U. S. 383, 389-390 (1944), because the state law would effectively rewrite an offense definition that Congress carefully considered, see Williams, 327 U. S., at 718, or because federal statutes reveal an intent to occupy so much of a field as would exclude use of the particular state statute at issue, see id., at 724 (no assimilation where Congress has "covered the field with uniform fed-
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