The state of Maryland passed a law requiring importers of foreign goods to obtain a license for selling their products.

[1] Chief Justice John Marshall delivered the opinion of the court, ruling that Maryland's statute violated the Import-Export and Commerce Clauses and the federal law was supreme.

He alleged that the power of a state to tax goods did not apply if they remained in their "original package".

[9] Marshall began with a lexicographical analysis of the Import-Export Clause: An impost, or duty on imports is a custom or a tax levied on articles brought into a country, and is most usually secured before the importer is allowed to exercise his rights of ownership over them, because evasions of the law can be prevented more certainly by executing it while the articles are in its custody.

[10] Marshall thus concluded that "th[e] exception in favor of duties for the support of inspection laws goes far in proving that the framers of the Constitution classed taxes of a similar character with those imposed for the purposes of inspection, with duties on imports and exports, and supposed them to be prohibited.

"[12] Regardless of whether that power was to prevent state taxation from disrupting harmony between the states, prevent states from hindering uniform trade relations between the US and foreign nations, or reserve this source of revenue solely to the government, "it is plain that the object would be as completely defeated by a power to tax the article in the hands of the importer the instant it was landed as by a power to tax it while entering the port.

[14] Maryland insisted that the point when the prohibition on taxation ends should be the time of importation, to which Marshall responded: It may be conceded that the words of the prohibition ought not to be pressed to their utmost extent; that in our complex system, the object of the powers conferred on the government of the Union, and the nature of the often conflicting powers which remain in the states, must always be taken into view, and may aid in expounding the words of any particular clause.

But while we admit that sound principles of construction ought to restrain all courts from carrying the words of the prohibition beyond the object the Constitution is intended to secure, that there must be a point of time when the prohibition ceases and the power of the state to tax commences, we cannot admit that this point of time is the instant that the articles enter the country.

[16] Marshall began this analysis by noting that "[t]he oppressed and degraded state of commerce previous to the adoption of the Constitution can scarcely be forgotten.

[18] The opinion concluded by noting: "It may be proper to add that we suppose the principles laid down in this case to apply equally to importations from a sister state.

In the nature of things, the line of division is in some degree vague and indefinite, and I do not see how it could be drawn more accurately and correctly, or more in harmony with the obvious intention and objected of this provision in the Constitution.

"[22] In 1860, Chief Justice Taney, wrote the Supreme Court's opinion in Almy v. California, which found a tax on a bill of lading for gold dust exported from California to New York violated the Import Export Clause.

[23] "We think this case cannot be distinguished from that of Brown v. Maryland,"[24] he wrote, concluding that "the state tax in question is a duty upon the export of gold and silver, and consequently repugnant to the [Import-Export Clause].

[29] In Michelin, the U.S. Supreme Court launched a sua sponte investigation of the meaning and purpose of the Import-Export Clause, summarizing it thusly: The Framers of the Constitution thus sought to alleviate three main concerns by committing sole power to lay imposts and duties on imports in the Federal Government, with no concurrent state power: the Federal Government must speak with one voice when regulating commercial relations with foreign governments, and tariffs, which might affect foreign relations, could not be implemented by the States consistently with that exclusive power; 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, with their crucial ports of entry, were prohibited from levying taxes on citizens of other States by taxing goods merely flowing through their ports to the other States not situated as favorably geographically.The Michelin Court made a lengthy and thorough analysis of the Brown opinion and how it was misread in Low v. Austin,[30] which "[held] that the Court in Brown included nondiscriminatory ad valorem property taxes among prohibited 'imposts' or 'duties.