Pacific Express Company v. Seibert‪.‬

According to the view we take of the case, it is not necessary to inquire into the special equities set forth in the bill and relied upon in the argument for complainant to show that this record presents a case for the interposition of a federal court, for the purpose of restraining the assessment or collection of a state tax. The primary and fundamental ground on which the maintenance of such a suit rests is the unlawfulness of the tax against which relief is sought, or, in other words, the invalidity or unconstitutionality of the legislative act under the authority of which the tax is imposed. It is true that this ground is not in itself sufficient. But when the illegality of the tax or the invalidity or unconstitutionality of the legislative act under which it is imposed is established, it becomes necessary to go further, and make out a case that can be brought under some recognized head of equity jurisdiction: such as, that the collection of the tax sought to be restrained may entail a multiplicity of suits; or cause some other irreparable injury, as, for instance, the ruin of complainants business; or, where the property is real estate, throw a cloud upon the title of the complainant. Shelton v. Platt, 139 U.S. 591, 594; Allen v. Pullmans Palace Car Co., 139 U.S. 658, 661.