Cite as: 539 U. S. 396 (2003)
Opinion of the Court
statute presented no evidence of a "specific interest of the Federal Government which might be interfered with" by the law. Id., at 459 (opinion concurring in result); see id., at 461 (finding "no evidence of adverse effect in the record"). He would, however, have found preemption in a case of "conflicting federal policy," see id., at 458-459, and on this point the majority and Justices Harlan and White basically agreed: state laws "must give way if they impair the effective exercise of the Nation's foreign policy," id., at 440 (opinion of the Court). See also Pink, 315 U. S., at 230-231 ("[S]tate law must yield when it is inconsistent with, or impairs . . . the superior Federal policy evidenced by a treaty or international compact or agreement"); id., at 240 (Frankfurter, J., concurring) (state law may not be allowed to "interfer[e] with the conduct of our foreign relations by the Executive").
It is a fair question whether respect for the executive foreign relations power requires a categorical choice between the contrasting theories of field and conflict preemption evident in the Zschernig opinions,11 but the question requires
11 The two positions can be seen as complementary. If a State were simply to take a position on a matter of foreign policy with no serious claim to be addressing a traditional state responsibility, field preemption might be the appropriate doctrine, whether the National Government had acted and, if it had, without reference to the degree of any conflict, the principle having been established that the Constitution entrusts foreign policy exclusively to the National Government. See, e. g., Hines v. David-owitz, 312 U. S. 52, 63 (1941). Where, however, a State has acted within what Justice Harlan called its "traditional competence," 389 U. S., at 459, but in a way that affects foreign relations, it might make good sense to require a conflict, of a clarity or substantiality that would vary with the strength or the traditional importance of the state concern asserted. Whether the strength of the federal foreign policy interest should itself be weighed is, of course, a further question. Cf. Rice v. Santa Fe Elevator Corp., 331 U. S. 218, 230 (1947) (congressional occupation of the field is not to be presumed "in a field which the States have traditionally occupied"); Boyle v. United Technologies Corp., 487 U. S. 500, 507-508 (1988) ("In an area of uniquely federal interest," "[t]he conflict with federal policy need not be as sharp as that which must exist for ordinary pre-emption").
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