Does federal law override state law? Yes, but only when the two actually conflict and Congress was acting within a power the Constitution gives it. Outside that zone, states keep broad authority to make their own rules, and even a valid federal law cannot force a state government to help enforce it. That is why marijuana can be legal in your state and illegal under federal law at the same time, and why a state can set stricter air pollution standards than Washington does.
The Constitutional Rule
Article VI, Clause 2 of the Constitution, the Supremacy Clause, declares that the Constitution, federal statutes enacted under it, and treaties made under U.S. authority are “the supreme Law of the Land,” and that judges in every state are bound by them regardless of anything in a state’s own constitution or laws.1Legal Information Institute. U.S. Constitution Article VI The Framers wrote this in because the Articles of Confederation had no way to make federal decisions stick. A state could simply ignore a federal law it disliked.
The clause does not mean everything Washington does automatically beats what a state does. It means that when a federal law is validly enacted and it collides with a state law, the federal law wins. Both halves of that sentence matter: the federal law has to be within the powers the Constitution grants Congress, and there has to be a real collision.
When Federal Law Actually Preempts State Law
The doctrine that puts the Supremacy Clause to work is called preemption. Courts sort it into a few categories, and which one applies often decides the case.
Express Preemption
The clearest situation is when Congress writes into the statute that it means to override state law. The Employee Retirement Income Security Act does this: its preemption clause states that the federal law supersedes “any and all State laws” that relate to covered employee benefit plans. When the text is that explicit, courts do not have to guess. The state law is out.
Field Preemption
Sometimes Congress regulates an area so completely that it has effectively claimed the whole subject, leaving no room for states to add their own rules even if those rules would complement the federal scheme. Immigration is the classic example. When the Supreme Court struck down most of Arizona’s SB 1070 in 2012, it held that provisions making it a state crime for noncitizens to fail to carry registration documents, and authorizing state officers to make warrantless arrests based on suspected immigration violations, intruded on a field the federal government had fully occupied.2Justia Law. Arizona v. United States, 567 U.S. 387 (2012)
Conflict Preemption
Conflict preemption applies when it is physically impossible to comply with both laws, or when the state law stands as an obstacle to what Congress was trying to accomplish. Congress does not have to have blanketed the entire field. A single state statute that frustrates the purpose of a single federal statute can be struck down.
The Default When the Statute Is Silent
If a federal law is ambiguous about whether it preempts state law, courts generally presume Congress did not intend to override state authority, especially in areas states have traditionally regulated. Whoever argues for preemption carries the burden.
What States Still Control on Their Own
The Tenth Amendment states that powers not delegated to the federal government, and not prohibited to the states, “are reserved to the States respectively, or to the people.”3Legal Information Institute. Tenth Amendment The federal government can only act where the Constitution gives it authority. Everything else belongs to the states or to individuals.
In practice, states run most of daily legal life through what lawyers call the police powers. States regulate family law, education, land use, most criminal conduct, professional licensing, contract disputes, and public health. Getting a driver’s license, sending a child to public school, buying a house, getting married, dealing with a zoning board: all state territory. The federal government generally cannot invade these areas without a clear constitutional hook such as the Commerce Clause or the Spending Clause.
The Federal Government Cannot Force States to Enforce Federal Law
Even where federal law is supreme, Congress cannot order state governments to do the enforcing. This is the anti-commandeering doctrine, and it explains some of the most visible federal-state standoffs in recent years.4Legal Information Institute. Anti-Commandeering Doctrine
The Supreme Court first articulated this in New York v. United States (1992), which struck down a federal law that essentially ordered states to take ownership of radioactive waste. Five years later, Printz v. United States ruled that Congress could not conscript state and local law enforcement officers to conduct background checks under the Brady Handgun Violence Prevention Act. In Murphy v. NCAA (2018), the Court struck down a federal law that prohibited states from authorizing sports gambling, calling that kind of directive “fundamentally incompatible with our constitutional system of dual sovereignty.”5Library of Congress. Murphy v. NCAA
The practical result: the federal government can make marijuana illegal under its own statutes and prosecute violations using federal agents, but it cannot order a state to criminalize marijuana or force state police to arrest people under federal drug law.
Where This Plays Out in Real Life
Marijuana
Cannabis remains a Schedule I controlled substance under federal law, the most restrictive category, reserved for drugs Congress has classified as having high abuse potential and no accepted medical use.6Office of the Law Revision Counsel. 21 USC 812 – Schedules of Controlled Substances Dozens of states have legalized it for medical use, recreational use, or both. A dispensary operating legally under state law is committing a federal crime.
The Supreme Court confirmed in Gonzales v. Raich (2005) that Congress has constitutional authority under the Commerce Clause to prohibit even locally grown, state-legal marijuana.7Library of Congress. Gonzales v. Raich, 545 U.S. 1 (2005) Federal law is technically supreme. But because of the anti-commandeering doctrine, the federal government cannot compel states to enforce that prohibition, so state-legal cannabis businesses live in a gray zone that depends on shifting federal enforcement priorities.
That gray zone has real financial consequences. Section 280E of the Internal Revenue Code bars any business trafficking in Schedule I or Schedule II substances from claiming standard business deductions or credits.8Office of the Law Revision Counsel. 26 USC 280E – Expenditures in Connection With the Illegal Sale of Drugs A state-legal dispensary can deduct the direct cost of the products it sells, but not rent, payroll, or advertising. Most major banks also refuse to serve cannabis businesses because handling drug proceeds is a federal crime.
In December 2025, President Trump signed an executive order directing the Attorney General to complete the process of moving marijuana from Schedule I to Schedule III.9The White House. Increasing Medical Marijuana and Cannabidiol Research If finalized, rescheduling would not legalize marijuana at the federal level, but it would end the Section 280E problem and open the door to recognized medical research. As of early 2026, the DEA rulemaking is still ongoing.
Immigration
Immigration is one of the clearest examples of field preemption. The federal government controls the admission, removal, and status of noncitizens through an extensive statutory and regulatory framework, and states cannot build parallel systems. In Arizona v. United States, the Court struck down three of four challenged SB 1070 provisions on that basis. The one provision it let survive, a requirement that state police check the immigration status of people lawfully stopped for other reasons, was read narrowly and left open to future challenges.
Environmental Regulation
Environmental law shows a different model. Rather than occupying the field, the Clean Air Act preserves state authority to adopt emission standards stricter than the federal baseline. The statute says nothing in it prevents a state from enforcing its own standards or pollution-control requirements, as long as they are at least as strict as the federal minimums.10Office of the Law Revision Counsel. 42 USC 7416 – Retention of State Authority States cannot go below the federal floor, but they can raise the ceiling. This cooperative federalism model uses states as laboratories for tougher rules while keeping a uniform national minimum.
When a State Law Burdens Interstate Commerce
Federal supremacy does not always require a federal statute. The Commerce Clause grants Congress power to regulate interstate commerce, and the Supreme Court reads it to contain an implied restriction on states even when Congress has said nothing. Under this dormant Commerce Clause, a state cannot pass a law that discriminates against or excessively burdens interstate commerce. A state that banned the import of out-of-state waste, or taxed interstate transactions more heavily than in-state ones, would face a constitutional challenge even without a competing federal statute. Courts weigh the burden on interstate commerce against the local benefit the state claims. If the burden clearly outweighs the benefit, the state law falls.
Challenging a State Law You Think Is Preempted
The usual path is a lawsuit in federal court. The challenger files a complaint arguing the state law violates the Supremacy Clause and typically asks for an injunction blocking enforcement. To get a preliminary injunction halting enforcement while the case proceeds, the challenger generally must show a likelihood of success on the merits and that enforcement in the meantime would cause irreparable harm.
These cases often reach the Supreme Court because lower courts in different parts of the country regularly disagree about whether a particular federal law preempts a particular state law. The same state statute may survive in one region and be struck down in another until the Court steps in. The process is slow and expensive, and most preemption disputes turn on close readings of statutory text and legislative history, where reasonable judges routinely disagree.
For individuals and businesses caught between the two systems, both laws apply until a court says otherwise. A cannabis business in a legal state still risks federal prosecution. A state environmental rule tougher than the federal baseline is valid unless a court decides Congress meant to cap it. A green light from one government does not guarantee safe passage from the other.