Answer:
This aspect of the Supremacy Clause reflected concerns that individual states were jeopardizing the fledgling nation’s security by putting the United States in violation of its treaty obligations. For instance, at the end of the Revolutionary War, Article IV of the Treaty of Peace between the United States and Great Britain had specified that “creditors on either side[] shall meet with no lawful impediment to the recovery of the full value in sterling money, of all bona fide debts heretofore contracted.” Nonetheless, several states enacted or retained debtor-relief laws whose enforcement against British creditors would violate this promise, and British diplomats argued that these violations excused Britain’s own failure to withdraw all armies and garrisons from the United States. The Supremacy Clause responded to this problem: just as state courts were not supposed to apply state laws that conflicted with the Constitution itself, so too state courts were not supposed to apply state laws that conflicted with Article IV of the Treaty of Peace. Indeed, the peculiar wording of the Supremacy Clause—covering treaties already “made . . . under the Authority of the United States” as well as treaties that “shall be made” in the future—was specifically designed to encompass pre-existing agreements like the Treaty of Peace.
