The '''Supremacy Clause''' ofis theessentially a [[Constitution conflict-of the United States-laws]] ([[Articlerule onespecifying ofthat thecertain Unitedfederal Statesacts Constitution#Supremacy|Articletake VI,priority Clauseover 21any )state establishesacts that theconflict Constitution,with [[Lawfederal oflaw. theSome Unitedjurists States|federalfurther laws]]argue madethat pursuantthe toclause it,also andnullifies [[Treaty|treaties]]federal madelaw underthat itsis authority,in constituteconflict with the "supremeConstitution, Lawalthough ofthis theis Land",disputed.<ref>{{Cite andweb|date=2012-04-13|title=The thusPriority takeof prioritythe Constitution over anyFederal conflictingStatutes [[- Mike Rappaport|url=https://constitutioncenterlawliberty.org/inConfederationthe-priority-of-the-constitution-over-federal-statutes/|access-date=2022-01-11|website=Law & Liberty|language=en-US}}</ref> The Supremacy Clause follows Article XIII of the [[Articles of Confederation]], the predecessor of the Constitution, which provided that "Every [[U.S. state|State]] shall abide by the determination of the [[Congress of the Confederation|[Congress]]], on all questions which by this confederation are submitted to them."<ref name=HGC62>{{cite web| last=Lawson| first=Gary| title=Essays on Article VI: Supremacy Clause| url=https://www.heritage.org/constitution/articles/6/essays/133/supremacy-clause| publisher=The Heritage Foundation| location=Washington D.C.| access-date=October 10, 2019}}</ref>

As a constitutional provision announcingidentifying the supremacy of federal law, the Supremacy Clause assumes the underlying priority of federal authority, albeit only when that authority is expressed in the Constitution itself;<ref>{{cite book|title=Fundamentals of American law|year=1998|chapter=Preemption Controversies|chapter-url=https://books.google.com/books?id=Si0lupMPrEoC&q=federal+preemption&pg=PA31 | first1=Alan B. | last1=Morrison | page=31 | isbn=978-0-19-876405-2 | publisher=Oxford University Press US}}</ref> no matter what the [[Federal government of the United States|federal]] or [[State governments of the United States|state governments]] might wish to do, they must stay within the boundaries of the Constitution.<ref>{{Cite web|date=2012-04-13|title=The Priority of the Constitution over Federal Statutes - Mike Rappaport|url=https://lawliberty.org/the-priority-of-the-constitution-over-federal-statutes/|access-date=2022-01-11|website=Law & Liberty|language=en-US|quote=During the ratification, various Federalists urged that "in pursuance" of the Constitution meant not just conformity with bicameralism and presentment, but otherwise consistent with the entire Constitution. Only such latter statutes were entitled to be treated as supreme over contrary state law. See, for example, Jensen, ed, 2 Documentary History of the Ratification at 517 (cited in note 59) (James Wilson commenting that “in pursuance” meant that a law was otherwise constitutional). Earlier, Wilson had claimed that Congress could not pass any laws restricting the press because such laws would not be in “pursuance” of the Constitution. Id at 455. See also Jonathan Elliot, ed, 4 The Debates in the Several State Conventions on the Adoption of the Federal Constitution 188 (2d ed 1836) (Governor Johnston of North Carolina commenting that every law consistent with the Constitution is “made in Pursuance” of it; those laws inconsistent are not made in Pursuance of it); id at 182 (William Davie commenting to the same effect); id at 28, 178–79 (James Iredell commenting to the same effect); Federalist 33 (Hamilton), in The Federalist 203, 207 (Wesleyan 1961) (Jacob E. Cooke, ed) (claiming that laws that are not pursuant to the Constitution, but instead invade state power, are acts of usurpation).}}</ref> Consequently, the Supremacy Clause is considered a cornerstone of the [[Federalism in the United States|United States' federal political structure]].<ref>{{cite book |first=W. Cleon |last= Skousen |title=The Making of America – The Substance and Meaning of the Constitution |url=https://archive.org/details/makingofamericas00skou |url-access=registration |publisher=National Center for Constitutional Studies |location=Washington D.C. |year=1985 |page=[https://archive.org/details/makingofamericas00skou/page/657 657]|isbn= 9780934364669 }}</ref><ref>{{Cite book|title=The Supremacy Clause: A Reference Guide to the United States Constitution|last=Drahozal|first=Christopher R.|date=2004|publisher=Greenwood Publishing Group|isbn=9780313314476|page=xiv|url=https://books.google.com/books?id=bAhi1GbCldsC&q=The+Supremacy+Clause:+A+Reference+Guide+to+the+United+States+Constitution}}</ref>

According to Madison's [[Notes of Debates in the Federal Convention of 1787]], the Supremacy Clause was introduced as part of the [[New Jersey Plan]].<ref name=Drahozal2004>{{Cite book|url=https://books.google.com/books?id=bAhi1GbCldsC&pg=PA16|title=The Supremacy Clause: A Reference Guide to the United States Constitution|last=Drahozal|first=Christopher R.|date=2004|publisher=Greenwood Publishing Group|isbn=9780313314476|page=16}}</ref><ref>{{Cite journal|last=LaCroix|first=Alison L.|date=May 2010|title=The Authority for Federalism: Madison's Negative and the Origins of Federal Ideology|url=https://www.cambridge.org/core/journals/law-and-history-review/article/the-authority-for-federalism-madisons-negative-and-the-origins-of-federal-ideology/401539AF1F045628F20D5325BE61A7F5|journal=Law and History Review|language=en|volume=28|issue=2|pages=451–505|doi=10.1017/S0738248010000064|s2cid=143794122|issn=1939-9022}}</ref> During the debate, it was first put up for a motion by [[Luther Martin]]<ref>{{Cite book|url=https://books.google.com/books?id=90VYAgAAQBAJ&pg=PA918|title=American International Law Cases Fourth Series: 2009|last=Board|first=Oceana Editorial|date=2011-01-06|publisher=Oceana|isbn=9780199758876|language=en}}</ref> on July 17th17, when it passed unanimously.<ref>{{Cite web|url=http://avalon.law.yale.edu/18th_century/debates_717.asp|title=Avalon Project - Madison Debates - July 17|website=avalon.law.yale.edu|access-date=2019-03-08}}</ref>

Alexander Hamilton, wrote in Federalist #No. 78 that, "There is no position which depends on clearer principles, than that every act of a delegated authority, contrary to the tenor of the commission under which it is exercised, is void. No legislative act, therefore, contrary to the Constitution, can be valid."<ref>{{Cite web|url=https://avalon.law.yale.edu/18th_century/fed78.asp|title=The Avalon Project : Federalist No 78|website=avalon.law.yale.edu|accessdate=May 15, 2021}}</ref>

''[[LULAC v. Wilson]]'' was brought to the Supreme Court in order to determine the constitutionality of California's Proposition 187, which the League of United Latin American Citizens argued was preempted by the federal government's authority over the regulation of foreign nationals in America. Proposition 187 was meant to assist cooperative efforts undertaken by national and sub-national governments to place stricter restrictions on undocumented immigrants "from receiving benefits or public services in the State of California". The Court decided that only a small portion of [[1994 California Proposition 187|PrepositionProposition 187]] was not preempted by the Personal Responsibility and Work Opportunity Reconciliation Act of 1996''.''<ref>{{Citation|title=League of United Latin American Citizens v. Wilson|date=November 14, 1997|url=https://scholar.google.com/scholar_case?case=1489340472264681690&q=LULAC+v.+Wilson&hl=en&as_sdt=400006|volume=997|pages=1244|access-date=2021-03-19}}</ref>

The supremacy of treaties over state law has been described as an "unquestioned axiom of the founding" of the United States.<ref>{{Cite web |date=2017-02-15 |title=The Origins and Fall of Treaty Supremacy and Its Significance |url=http://opiniojuris.org/2017/02/15/32999/ |access-date=2023-04-07 |website=Opinio Juris |language=en-US}}</ref> Under the Supremacy Clause, treaties and federal statutes are equally regarded equally as "supreme law of the land" with "no superior efficacy ... given to either over the other".<ref>"[https://www.law.cornell.edu/supremecourt/text/124/190 WHITNEY et al. v. ROBERTSON, Collector]". ''LII / Legal Information Institute''. Retrieved December 18, 2021.</ref> Thus, international agreements made pursuant to the [[Treaty Clause]]—namely, ratified with the [[advice and consent]] of a two-thirds [[supermajority]] of the [[United States Senate|Senate]]—are treaties in the constitutional sense and thereby incorporated into [[United States Code|U.S. federal law]] no differently than an [[Legislation|act of Congress]]. Treaties are likewise subject to [[judicial interpretation]] and [[Judicial review|review]] just as any federal statute, and courts have consistently recognized them as legally binding under the Constitution.

The [[Supreme Court of the United States|U.S. Supreme Court]] applied the Supremacy Clause for the first time in the 1796 case, ''[[Ware v. Hylton]],'' ruling that a treaty superseded conflicting state law.<ref>3 U.S. (3 Dall.) 199 (1796)</ref> The Court held that both states and private citizens were bound to comply with the treaty obligations of the federal government, which was in turn bound by the "[[International law|law of nations]]" to honor treaties. Shortly thereafter, in the 1801 case, ''[[United States v. The Schooner Peggy]],'' the court ruled in favor of a private citizen's lawsuit against the government on the basis of a treaty, and for the first time elaborated upon supreme nature of ratified treaties:<blockquote>[W]here a treaty is the law of the land, and as such affects the rights of parties litigating in court, that treaty as much binds those rights and is as much to be regarded by the court as an act of congress; and although restoration may be an executive, when viewed as a substantive act, independent of and unconnected with other circumstances, yet to condemn a vessel, the restoration of which is directed by a law of the land, would be a direct infraction of that law, and of consequence improper.</blockquote>In ''[[Foster v. Neilson|Foster v. Nielson]]'' (1829), [[John Marshall|Chief Justice John Marshall]], writing for the majority, affirmed that a treaty is constitutionally the "law of the land", but for the first time articulated thea differencedistinction between self-executing and non-self-executing agreements with respect to domestic law:<blockquote>Our constitution declares a treaty to be the law of the land. It is, consequently, to be regarded in courts of justice as equivalent to an act of the legislature, whenever it operates of itself, without the aid of any legislative provision. But when the terms of the stipulation import a contract—when either of the parties engages to perform a particular act, the treaty addresses itself to the political, not the judicial department; and the legislature must execute the contract, before it can become a rule for the court.</blockquote>While it is generally agreed by constitutional scholars that treaties are as binding as domestic federal law, courts have differed on the enforceability of some types of international agreements and on the precise scope of a treaty's legal obligations. Beginning with the 1884 [[Head Money Cases]], the Supreme Court has consistently held that Congress can abrogate a treaty by legislative action even if this amounts to a violation of the treaty under international law; indeed, courts will enforce congressional modifications of a treaty regardless of whether foreign actors still consider the treaty to be binding on the U.S. Nevertheless, in ''[[Missouri v. Holland]]'' (1920), the Supreme Court held that the Supremacy Clause allows the federal government to make [[treaty|treaties]] that supersede state law even if such treaties might abrogate states' rights arising under the [[Tenth Amendment to the United States Constitution|Tenth Amendment]].<ref>[[Case citation|252 U.S. 416]]</ref> The decision implied that treaties can be used to legislate in areas otherwise within the exclusive authority of the states, and by extension in areas not within the scope of the federal government or its branches.

However, ''Missouri''<nowiki/>'s potentially broad interpretation was circumscribed in the 1957 case, ''[[Reid v. Covert]],'' when the Supreme Court held that treaties and the laws made pursuant to them must comply with the Constitution. The enforceability of treaties was further limited in the 2008 Supreme Court decision in ''[[Medellín v. Texas]]'', which held that even if a treaty may constitute an international commitment, it is not binding domestic law unless it has been implemented by an act of Congress or is itself explicitly "[[Self-executing right|self-executing]]".<ref>"[https://www.law.cornell.edu/constitution-conan/article-2/section-2/clause-2/the-treaty-making-power The Treaty Making Power]". ''LII / Legal Information Institute''. Retrieved December 19, 2021.</ref> Law scholars called the ruling "an invisible constitutional change" that departed from both longtime historical practice and the plain language of the Supremacy Clause.<ref>{{Cite web|date=February 15, 2017|title=The Origins and Fall of Treaty Supremacy and Its Significance|url=http://opiniojuris.org/2017/02/15/32999/|accessdate=May 15, 2021}}</ref>

However, in the case of ''[[California v. ARC America Corp.]]'', {{ussc|490|93|1989}}, the Supreme Court held that if Congress expressly ''intended'' to act in an area, this would trigger the enforcement of the Supremacy Clause, and hence nullify the state action. The Supreme Court further found in ''[[Crosby v. National Foreign Trade Council]]'', {{ussc|530|363|2000}}, that even when a state law is not in direct conflict with a federal law, the state law could still be found unconstitutional under the Supremacy Clause if the "state law is an obstacle to the accomplishment and execution of Congress's full purposes and objectives".<ref>''Crosby v. National Foreign Trade Council'', 530 U.S. 363, 372-374.</ref> Congress need not expressly assert any preemption over state laws either, because Congress may implicitly assume this preemption under the Constitution.<ref>''Crosby v. National Foreign Trade Council'', 530 U.S. 363, 386-388.</ref> Finally, in [[Murphy v. National Collegiate Athletic Association]] the Supreme Court enforced the Supremacy Clause by overturning Federal law as an unconstitutional encroachment into the domain of the states not within of the limits of the [[Delegated powers]], stating that "''The Constitution confers on Congress not plenary legislative power but only certain enumerated powers''".