¹ “A container is a permanent reusable article of transport equipment . . . durably made of metal, and equipped with doors for easy access to the goods and for repeated use. It is designed to facilitate the handling, loading, stowage aboard ship, carriage, discharge from ship, movement, and transfer of large numbers of packages simultaneously by mechanical means to minimize the cost and risks of manually processing each package.“ Simon, The Law of Shipping Containers, 5 J. Maritime L. & Comm. 507, 513 (1974).

See Customs Convention on Containers, Art. I (b), May IS, 1956, [1969] 20 U.S.T. 301, 304, T.I.A.S. No. 6634. Although containers may be as small as 1 cubic meter (35.3 cubic feet), 49 CFR §420.3 (c)(5) (1977), they are typically S feet high, S feet wide, and between S and 40 feet long. Simon, at 510.

² The opinion of the Superior Court is not officially reported.

³ The Court of Appeal also rejected, 132 Cal. Rptr., at 534, appellants' argument that California's tax was prohibited by Art. XI, §§ 1 & 4, and by Art. XXII, § 2, of the Treaty of Friendship, Commerce and Navigation Between the United States of America and Japan, Apr. 2, 1953, [1953] 4 U. S. T. 2063, T. I. A. S. No. 2863 (providing that Japanese nationals residing in the U. S. may not be subjected to payment of taxes “more burdensome than those borne by” United States nationals, and according Japan “most favored nation” status). Appellants repeat this argument here, and we reject it. The provisions appellants cite interdict discrimination against Japanese nationals; there is no evidence that California has treated Japanese containers differently from domestic containers for purposes of applying its property tax.

The Court of Appeal likewise rejected, 132 Cal. Rptr., at 533, appellants' argument that California's tax constituted an indirect “duty of Tonnage” proscribed by U. S. Const., Art. I, § 10, cl. 3. Appellants repeat this argument here; in view of our disposition, we do not reach it. The Court of Appeal noted that appellants did not challenge California's tax on due process grounds. See 132 Cal. Rptr., at 532 n. 2. Although appellants proffer a due process challenge here, we need not reach it either.

⁴ The California Supreme Court also rejected appellants' argument that California's tax constituted “Imposts or Duties” proscribed by U. S. Const., Art. I, § 10, cl. 2. 20 Cal. 3d, at 186-188, 571 P. 2d, at 258-259. Appellants reiterate this argument here; in view of our disposition, we do not consider it. In their petition for rehearing, appellants argued that the tax contravened Art. III, §§ 1 & 2 of the General Agreement on Tariffs and Trade (GATT), 61 Stat. A3, A18 (1947) (providing that “imported products” may not be subjected to heavier taxes, or to less favorable treatment, than like products of domestic origin). Petition for Rehearing 35-40. The court rejected this latter argument sub silentio. 20 Cal. 3d, at 190. Appellants repeat this argument here, and we deem it frivolous. Assuming arguendo that appellants' containers, as instrumentalities of commerce entering this country subject to re-exportation, could be labeled “imported products” within the meaning of GATT, the provisions on which appellants rely prohibit only discriminatory treatment. As noted above, supra n. 3, there is no evidence that California has treated Japanese containers differently from domestic containers for purposes of applying its property tax.

⁵ The “home port doctrine” was reaffirmed, as to ocean-going vessels, in Morgan v. Parham, 16 Wall. 471, 476-477 (1872), and in Southern Pac. Co. v. Kentucky, 222 U. S. 63, 69 (1911). It was applied to vessels moving in inland waters in St. Louis v. Ferry Co., 11 Wall. 423 (1870), and in Ayer & Lord Tie Co. v. Kentucky, 202 U. S. 409, 421-423 (1906).

⁶ See, e. g., Note, 49 Cal. L. Rev. 968, 970-971 (1961); Note, State Taxation of International Air Transportation, 11 Stan. L. Rev. 518, 522, and n. 19 (1959); Page, Jurisdiction to Tax Tangible Movables, 1945 Wis. L. Rev. 125,143-144.

⁷ Accordingly, we do not reach questions as to the taxability of foreignowned instrumentalities engaged in interstate commerce, or of domesticallyowned instrumentalities engaged in foreign commerce. Cf. Sea-Land Service, Inc. v. County oj Alameda, 12 Cal. 3d 772, 528 P. 2d 56 (1974) (domestically-owned containers used in intercoastal and. foreign commerce held subject to apportioned property tax); Flying Tiger Line, Inc. v. County of Los Angeles, 51 Cal. 2d 314, 333 P. 323 (1958) (domesticallyowned aircraft used in foreign commerce held subject to apportioned property tax).

⁸ By taxing property present on the “lien date,” California roughly apportions its property tax for mobile goods like containers. For example, if each of appellants' containers is in California for three weeks a year, the number present on any arbitrarily selected date would be roughly 3/52 of the total entering the State that year. Taxing 3/52 of the containers at full value, however, is the same as taxing all the containers at 3/52 value. Thus, California effectively apportions its tax to reflect the containers' “average presence,” i. e., the time each container spends in the State per year.

⁹ As noted above, the trial court found that appellants' containers are “instrumentalities of foreign commerce” that are “used constantly and exclusively for the transportation of cargo for hire in foreign commerce.” App. 35, 36.

¹⁰ Appellants' containers entered the United States pursuant to the Customs Convention on Containers, see n. 1, supra, which grants containers “temporary admission free of import duties and import taxes and free of import prohibitions and restrictions,” provided they are used solely in foreign commerce and are subject to re-exportation. 20 U. S. T., at 304. Similarly, 19 CFR § 10.41a (a)(3) (1978) designates containers “instruments of international traffic,” with the result that they “may be released without entry or the payment of duty” under 19 U. S. C. § 1322 (a). See 19 CFR § 10.41a (a)(1) (1978). A bilateral tax convention between Japan and the United States associates containers with the vehicles that carry them, and provides that income “derived by a resident of a Contracting State . . . from the use, maintenance, and lease of containers and related equipment . . . in connection with the operation in international traffic of ships or aircraft . . . is exempt from tax in the other Contracting State.” Convention Between the United States of America and Japan' for the Avoidance of Double Taxation, Mar. 8, 1971, [1972] 23 U. S. T. 967,1084-1085, T. I. A. S. No. 7365.

¹¹ Ocean-going vessels, for example, are generally taxed only in their nation of registry; this fact in part explains the phenomenon of “flags of convenience” (a term deemed derogatory in some quarters), whereby vessels are registered under the flags of countries that permit the operation of ships “at a nominal level of taxation.” See B. Boczek, Flags of Convenience 5, 56-57 (1962). Aircraft engaged in international traffic, apparently, are likewise “subject to taxation on an unapportioned basis by their country of origin.” Note, 11 Stan. L. Rev., at 519, and n. 11. See, e. g., SAS, 56 Cal. 3d, at 17, and n. 3, 363 P. 2d, at 28, and n. 3.

¹² E. g., The Federalist No. 42, pp. 279-283 (J. Cooke ed. 1961) (Madi-son); 3 M. Farrand, The Records of the Federal Convention of 1787, at 478 (1911) (Madison). See Note, State Taxation of International Air Carriers, 57 Nw. U. L. Rev. 92, 101, and n. 42 (1962); Note, 11 Stan. L. Rev., see n. 6, supra, at 525-526, and n. 29; Abel, The Commerce Clause in the Constitutional Convention and in Contemporary Comment, 25 Minn. L. Rev. 432, 465-475 (1941) (concluding, after an exhaustive survey of contemporary materials: “Despite the formal parallelism of the grants, there is no tenable reason for believing that anywhere nearly so large a range of action was given over commerce ‘among the several states’ as over that ‘with foreign nations.’” Id., at 475).

¹³ E. g., Butt field v. Stranahan, 192 U. S. 470, 492-493 (1904) (“exclusive and absolute” power of Congress over foreign commerce); Bowman v. Chicago & N. R. Co., 125 U. S. 465, 482 (18S8) (“It may be argued [that] the inference to be drawn from the absence of legislation by Congress on the subject excludes state legislation affecting commerce with foreign nations more strongly than that affecting commerce among the States. Laws which concern the exterior relations of the United States with other nations and governments are general in their nature, and should proceed exclusively from the legislative authority of the nation.”) ; Henderson v. Mayor of New York, 92 U. S. 259, 273 (1875) (regulation “must of necessity be national in its character” when it affects “a subject which concerns our international relations, in regard to which foreign nations ought to be considered and their rights respected.”); Gibbons v. Ogden, 9 Wheat. 1, 228-229 (1824) (Johnson, J., concurring). See also Atlantic Cleaners & Dyers, Inc. v. United States, 286 U. S. 427, 434 (1932). In National League of Cities v. Usery, 426 U. S. 833 (1976), the Court noted that Congress' power to regulate interstate commerce may be restricted by considerations of federalism and state sovereignty. It has never been suggested that Congress' power to regulate foreign commerce could be so limited.

¹⁴ The policies animating the Import-Export Clause and the Commerce Clause are much the same. In Michelin, the Court noted that the Import-Export Clause met three main concerns: “[T]he Federal Government must speak with one voice when regulating commercial relations with foreign governments . . . ; import revenues were to be the major-source of revenue of the Federal Government and should not be diverted to the States; and harmony among the States might be disturbed unless seaboard States . . . were prohibited from levying taxes on [goods in transit].” 423 U. S., at 285-286 (footnotes omitted). Abel, see n. 12, supra, observed that the Commerce Clause was directed to similar concerns. See 25 Minn. L. Rev., at 448, and n. 67, 452, and n. 81, 456-457, and n. 110 (need to deal in unified manner with foreign nations); id., at 446-451 (need to preserve federal revenue); id., at 448-449, and nn. 69-70, 470-471, 472-473 (need to prevent disharmony among States on account of import duties). In Washington Revenue Dept., supra, we noted that the third Michelin factor—preserving harmony among the States—mandated the same inquiry as to the effect of a state tax as the interstate Commerce Clause. See 435 U. S., at 754-755. In this case, similarly, the first Michelin factor—the need to speak with one voice when regulating commercial relations with foreign governments—mandates the same inquiry as to the effect of a state tax as the foreign Commerce Clause. In Washington Revenue Dept., the Court, holding that the state tax at issue did not prevent “speaking with one voice,” noted: “No foreign business or vessel is taxed.” 435 U. S., at 754.

¹⁵ See Developments in the Law—Federal Limitations on State Taxation of Interstate Business, 75 Harv. L. Rev. 953, 9S6 (1962) (noting the difficulty of allocating “international bridge time” for aircraft engaged in international commerce, with consequent risk of multiple taxation from overlapping apportionment formulae, and concluding that apportioned state taxation of foreign-owned aircraft should be forbidden).

¹⁶ Cf. Chy Lung v. Freeman, 92 U. S. 275, 279 (1875) (invalidating California's bond requirement for Chinese immigrants):

“[I]f this plaintiff and her twenty companions had been subjects of the Queen of Great Britain, can any one doubt that this matter would have been the subject of international inquiry, if not of a direct claim for redress? Upon whom would such a claim be made? Not upon the State of California; for, by our Constitution, she can hold no exterior relations with other nations. It would be made upon the government of the United States. If that government should get into a difficulty which would lead to war, or to suspension of intercourse, would California alone suffer, or all the Union?”

¹⁷ The stipulation of facts, App. 32, like the trial court's finding, id., at 35, states that “[a]ll containers of [appellants] are subject to property tax and are, in fact, taxed in Japan.” The record does not further elaborate on the nature of Japan's property tax. Appellants have uniformly insisted, Brief 9, Tr. of Oral Arg. 3, that Japan's property tax is unapponioned, ;'. e., that it is imposed on the containers' full value, and we so understand the trial court's finding. Although appellees do not seriously challenge this understanding, Brief 10-11, and n. 2, amicus curiae Multistate Tax Commission suggests that the record is inadequate to establish double taxation in fact: Japan, amicus says, may offer “credits . . . for taxes paid elsewhere.” Brief 8. Amicus provides no evidence to support this theory. Both the Solicitor General, Brief for United States as Amicus Curiae 19 n. 9, and the Department of State, id., at 17a, assure us that Japan taxes appellants' containers at their “full value,” and we accept this interpretation of the trial court's factual finding.

Because California's tax in this case creates multiple taxation in fact, we have no occasion here to decide under what circumstances the mere risk of multiple taxation would invalidate a state tax, or whether this risk would be evaluated differently in foreign, as opposed to interstate, commerce. Compare Moorman Mjg. Co. v. Bair, 437 U. S. 267, 276-277 (1978), and Washington Revenue Dept., 435 U. S., at 746, with, e. g., Central R. Co., 370 U. 3., at 615; Ott, 336 U. S., at 175; and Northwest Airlines, 322 U. S., at 326 (Stone, C. J., dissenting).

¹⁸ Retaliation by some nations could be automatic. West Germany's wealth tax statute, for example, provides an exemption for foreign-owned instrumentalities of commerce, but only if the owner's country grants a reciprocal exemption for German-owned instrumentalities. Vermogensteuergesetz (VStG) §2, ¶ 3, reprinted in I Bundesgcsetzblatt (BGB1) 949 (Apr. 23, 1974). The European Economic Community (EEC), when apprised of California's tax on foreign-owned containers, apparently determined to consider “suitable counter-measures.” Press Release, 521st Council Meeting—Transport (Luxembourg, June 12, 1978), p. 21.

¹⁹ Ore. Op. Atty. Gen. No. 7709 (Jan. 31, 1979) (citing decision below).

²⁰ Appellees' reliance on Bob-Lo Excursion Co. v. Michigan, 333 U. S. 28 (1948), is also misplaced. In that case, the appellant, a Michigan corporation, transported passengers from Detroit to an amusement park on an island in the Province of Ontario; the appellant refused to accept Negro passengers and was prosecuted under a Michigan civil rights statute. In sustaining the statute's application against Commerce Clause attack, the Court emphasized that the appellant conducted “foreign commerce” in name only. The sole business on the island was the amusement park, and it catered solely to American patrons. There were “no established means of access from the Canadian shore to the island,” id., at 36, and the island was “economically and socially . . . an amusement adjunct of the city of Detroit.” Id., at 35. The “highly closed and localized manner” in which the business was run insulated it “from all commercial or social intercourse and traffic with the people of another country usually characteristic of foreign commerce.” Id., at 36. The Court noted that the possibility of conflicting Canadian regulation was “so remote that it [was] hardly more than conceivable,” id., at 37, and concluded that, on the facts of the case, it was “difficult to imagine what national interest or policy, whether of securing uniformity in regulating commerce affecting relations with foreign nations or otherwise, could reasonably be found to be adversely affected by applying Michigan's statute to these facts or to outweigh her interest in doing so.” Id., at 40.

Bob-Lo is consistent with both the analysis and the result in the present case. Whereas in Bob-Lo the risk that foreign commerce would be burdened by inconsistent international regulation was “remote,” the risk that foreign commerce will be burdened by international multiple taxation here has been realized in fact. And whereas the Michigan statute posed no threat at all to the Federal Government's ability to “speak with one voice” in regulating foreign trade, the impairment of federal uniformity worked by California's statute is substantial