All US laws have to be consistent with international law – international law is part of the plan process
Bradly 15, William Van Alstyne Professor at Duke Law School and is the author of International Law in the U.S. Legal System, Second Edition (OUP 2015). He currently serves as a Reporter for the American Law Institute’s Restatement (Fourth) project on the Foreign Relations Law of the United States, and is a member of the Board of Editors of the American Journal of International Law. (Curtis, 05-18-2015, "Customary international law’s uncertain status in the US Legal System", Oxford University Press blog, https://blog.oup.com/2015/05/customary-international-laws-uncertain-status-in-the-us-legal-system///orakng)
Since Erie, the federal courts have developed discrete bodies of “federal common law” – that is, judicially developed rules that have the status of federal, national law (and which, therefore, displace contrary state law). The Supreme Court held in its 1964 Sabbatino decision, for example, that the “act of state doctrine,” whereby courts are to treat as valid the acts of foreign sovereigns taken within their territories, is a rule of federal common law binding on state courts. Some commentators have argued that all of customary international law should, like the act of state doctrine, be viewed as having the status of federal common law. If accepted, this argument would mean that courts would have broad authority to invalidate state laws deemed inconsistent with emerging norms of customary international law – perhaps, for example, concerning the death penalty. It might even mean that courts could enforce customary international law against the executive branch, and potentially even use it (as they can self-executing treaties) to override earlier-in-time federal statutes.