Congress sought to prevent the import of gray-market goods when it enacted § 526 of the Tariff Act of 1930, which prohibited the import of foreign-manufactured merchandise containing a trademark owned by a United States citizen or entity. The United States Customs Service (Customs) issued a regulatory rule that created exceptions to § 526, allowing the import of goods that would otherwise be banned. Cartier (plaintiff) was an association of trademark holders. Cartier brought a declaratory-judgment action to have the Customs rule invalidated for being inconsistent with § 526. K Mart (defendant) intervened to defend the rule. The district court upheld the rule. The court of appeals reversed. The United States Supreme Court granted certiorari.