The supreme law of the land in the United States is the Constitution, together with the federal statutes passed under it and the treaties made by the United States. Article VI, Clause 2 of the Constitution says so directly, and it adds that state judges are bound by these three sources of federal authority even when their own state’s constitution or laws say otherwise.1Legal Information Institute at Cornell Law School. U.S. Constitution Article VI When a state rule conflicts with any of the three, the state rule loses.
What Article VI Actually Says
The relevant language sits in a single sentence known as the Supremacy Clause. It does three things at once. It makes the Constitution itself supreme. It elevates federal statutes enacted under constitutional authority. And it gives treaties the same binding force as the Constitution and federal law.1Legal Information Institute at Cornell Law School. U.S. Constitution Article VI
The clause is short, but it settled a fight that had nearly wrecked the country’s first attempt at self-government. It also gave every future dispute between Washington and the states a fixed rule of decision: if the federal side is acting within its constitutional powers, its law wins.
The first great test came in McCulloch v. Maryland in 1819, when Maryland tried to tax the Second Bank of the United States. Chief Justice John Marshall ruled that the bank was a lawful exercise of implied congressional power and that no state could tax a federal institution. Marshall wrote that “the States have no power, by taxation or otherwise, to retard, impede, burden, or in any manner control, the operations of the constitutional laws enacted by Congress.”2National Archives. McCulloch v. Maryland (1819) That decision fixed the practical meaning of federal supremacy.
The Hierarchy Inside the Supreme Law
Not everything called “supreme law” sits at the same level. The Constitution itself is the highest authority. Federal statutes and treaties rank below the Constitution but above any state law. The Supremacy Clause draws this distinction by qualifying that only federal laws made “in Pursuance” of the Constitution qualify as supreme.3Constitution Annotated, Congress.gov. ArtVI.C2.1 Overview of Supremacy Clause A federal statute that violates the Constitution is void, no matter how many votes it received in Congress.
That principle was established in Marbury v. Madison (1803), when Chief Justice Marshall declared that “a Law repugnant to the Constitution is void.” The decision created judicial review, giving federal courts authority to strike down federal and state laws that conflict with the Constitution.4National Archives. Marbury v. Madison (1803)
Federal statutes and treaties sit on the same level as each other. When the two conflict, courts apply what is known as the last-in-time rule: whichever was enacted or ratified more recently controls. A treaty can override an earlier federal statute, as the Supreme Court recognized in Cook v. United States when a 1924 treaty superseded the Tariff Act of 1922. A later statute can likewise override an earlier treaty.5Legal Information Institute at Cornell Law School. Legal Effect of Treaties on Prior Acts of Congress
Treaties come with one important complication. A self-executing treaty becomes enforceable domestic law the moment it is ratified. A non-self-executing treaty creates international obligations for the United States but does not become enforceable in U.S. courts unless Congress passes implementing legislation. The Supreme Court drew this line in Medellin v. Texas (2008), holding that treaties “are not domestic law unless Congress has either enacted implementing statutes or the treaty itself conveys an intention that it be self-executing.”6Justia Law. Medellin v. Texas, 552 U.S. 491 (2008) The last-in-time rule applies only to self-executing treaties.5Legal Information Institute at Cornell Law School. Legal Effect of Treaties on Prior Acts of Congress
Treaties can also expand what Congress is able to do domestically. In Missouri v. Holland (1920), the Supreme Court upheld the Migratory Bird Treaty Act on the ground that treaties made under the authority of the United States are supreme law and that Congress may pass whatever legislation is necessary to carry them out.7Justia Law. Missouri v. Holland, 252 U.S. 416 (1920)
What Happens When State Law Conflicts
When a federal law displaces a state law, lawyers call it preemption. It shows up in three forms, and the differences matter because they decide how much room states have to legislate alongside Congress.
- Express preemption. Congress writes into a statute that federal law overrides state law on a particular subject. No ambiguity: the statute itself says state rules do not apply.
- Field preemption. Congress regulates an area so thoroughly that the scope of the federal scheme leaves no room for state law, even without a direct conflict. Immigration is the classic example.
- Conflict preemption. A state law directly contradicts a federal requirement, or it is impossible to comply with both at the same time. The state law falls.
Conflict preemption showed up in the Supreme Court’s docket as early as 1824, in Gibbons v. Ogden, when New York’s steamboat monopoly on its waters clashed with a federal shipping license. Congress’s power over interstate commerce won, and the state monopoly fell.
The Civil Rights Act of 1964 is one of the most consequential examples of express preemption. The law prohibited discrimination in public accommodations, schools, and employment based on race, color, religion, sex, or national origin, and it overrode state and local laws that permitted or required segregation.8National Archives. Civil Rights Act (1964) States could still pass their own anti-discrimination laws, but they could no longer pass laws that required or allowed practices the federal statute banned.9U.S. Equal Employment Opportunity Commission. Title VII of the Civil Rights Act of 1964
Where Federal Supremacy Stops
Federal supremacy is not unlimited. The Constitution grants specific powers to the national government, and the Tenth Amendment states that everything else stays with the states or the people.10Legal Information Institute at Cornell Law School. Tenth Amendment Several doctrines enforce those boundaries.
The Anti-Commandeering Doctrine
Congress can pass federal laws and enforce them through federal agencies, but it cannot order state legislatures to pass laws or draft state officials into administering a federal program. The Supreme Court announced the rule in New York v. United States (1992) and extended it in Printz v. United States (1997), striking down a provision of the Brady Act that required local sheriffs to conduct background checks on gun buyers. Justice Scalia wrote that “the Federal Government may neither issue directives requiring the States to address particular problems, nor command the States’ officers to administer or enforce a federal regulatory program.”11Legal Information Institute at Cornell Law School. Tenth Amendment – Anti-Commandeering Doctrine
The doctrine reached sports betting in Murphy v. NCAA (2018), when the Court struck down a federal law prohibiting states from legalizing sports gambling. Congress had not made sports gambling a federal crime; it had told states they could not authorize it. The Court found the two indistinguishable: “The distinction between compelling a State to enact legislation and prohibiting a State from enacting new laws is an empty one.”12Supreme Court of the United States. Murphy v. National Collegiate Athletic Association (2018)
The Limits of Spending Power
What Congress cannot command, it often achieves through money, attaching conditions to federal funding that states must follow if they want the funds. The Supreme Court has upheld this practice but imposed limits: the spending must serve the general welfare, the conditions must be clearly stated, they must relate to the federal interest in the program, and they cannot induce states to violate the Constitution.13Constitution Annotated, Congress.gov. General Welfare, Relatedness, and Independent Constitutional Bars
There is also a line between encouragement and coercion. The Court drew that line in NFIB v. Sebelius (2012), the Affordable Care Act case. Congress had threatened to strip all existing Medicaid funding from states that refused to expand their Medicaid programs. Seven justices found the threat unconstitutionally coercive, leaving states free to decline the expansion without losing their existing Medicaid money.
How the Rule Plays Out Today
The Supremacy Clause looks clean on paper. Real conflicts are messier and sometimes drag on for years.
Marijuana
Federal law still classifies marijuana as a Schedule I controlled substance, the same category as heroin, defined as having no accepted medical use and a high potential for abuse.14United States Drug Enforcement Administration. Drug Scheduling Over 40 states plus the District of Columbia have established regulated medical marijuana programs, and a growing number allow recreational use.15The White House. Increasing Medical Marijuana and Cannabidiol Research Under a strict reading of the Supremacy Clause, federal law should preempt those state programs. In practice, the federal government has largely declined to enforce its prohibition against users and businesses complying with state law, creating a gray zone that has lasted over a decade. A federal rescheduling process is underway as of early 2026.
Immigration
Immigration is where field preemption hits hardest. In Arizona v. United States (2012), the Supreme Court struck down three major provisions of Arizona’s SB 1070, finding that making it a state crime to be in the country without authorization, criminalizing unauthorized work, and authorizing warrantless arrests based on suspected deportable status all conflicted with the comprehensive federal immigration scheme.16Legal Information Institute at Cornell Law School. Arizona v. United States Only a narrow “show me your papers” provision survived. States cannot build their own parallel immigration enforcement systems.
Minimum Wage
Not every federal-state difference is a conflict. The federal minimum wage sits at $7.25 per hour and has not changed since 2009.17U.S. Department of Labor. State Minimum Wage Laws Most states have set higher rates, with some exceeding $16 per hour. This is legal because the federal rate is a floor, not a ceiling. The same principle applies across many areas of labor and consumer law. The Supremacy Clause only kicks in when state law contradicts federal law, not when it goes further in the same direction.
Who Decides
Courts are what make the Supremacy Clause enforceable rather than aspirational. Without judicial review, a state could ignore federal law and dare anyone to stop it.
Judicial review, established in Marbury v. Madison, lets federal courts strike down any law, federal or state, that conflicts with the Constitution. When the target is a state law, courts analyze whether Congress preempted the field, whether the state law directly conflicts with federal requirements, or whether compliance with both is impossible.
When a state official is actively enforcing an unconstitutional law, federal courts can issue an injunction stopping them, even though the Eleventh Amendment generally bars lawsuits against states. The Supreme Court carved out this exception in Ex parte Young (1908), reasoning that an official enforcing an unconstitutional statute acts “without the authority” of the state and can be sued personally.18Federal Judicial Center. Ex parte Young Separately, federal law lets individuals sue state officials who violate their federal constitutional rights, creating a private enforcement mechanism that does not depend on the federal government choosing to act.19Office of the Law Revision Counsel. 42 U.S. Code 1983 – Civil Action for Deprivation of Rights
The Constitution, federal statutes made under it, and U.S. treaties always outrank state law. But whether a specific federal rule reaches a specific state situation depends on which of the three is doing the work, whether Congress meant to preempt the field, and whether the federal government has stayed within the powers the Constitution actually grants it. Those questions get answered case by case, and the answers keep moving.