The treaty-power cannot purport to amend the Constitution by adding to the list of Congress’s enumerated powers, but having acted, the consequence will often be that it has provided Congress with an opportunity to enact measures that independently of a treaty Congress could not enact; the only question that can be raised as to such measures is whether they are “necessary and proper” for the carrying of the treaty in question into operation.

The foremost example of this interpretation is Missouri v. Holland. There, the United States and Great Britain had entered into a treaty for the protection of migratory birds, and Congress had enacted legislation pursuant to the treaty to effectuate it. Missouri objected that such regulation was reserved to the states by the Tenth Amendment and that the statute infringed on this reservation, pointing to lower court decisions voiding an earlier act not based on a treaty. Noting that treaties “are declared the supreme law of the land,” Justice Holmes for the Court said: “If the treaty is valid there can be no dispute about the validity of the statute under Article I, § 8, as a necessary and proper means to execute the powers of the Government.”

https://www.law.cornell.edu/constitution-conan/article-2/section-2/clause-2/treaties-as-law-of-the-l...