States can enforce federal law, but only in specific circumstances: when they pass their own laws covering the same conduct, when they volunteer to cooperate with federal agencies, or when Congress has specifically granted them enforcement authority. What the federal government cannot do is force a state to enforce federal law. That boundary, drawn by the Tenth Amendment and a handful of Supreme Court decisions, controls almost every dispute you’ll see about immigration, drug policy, gun regulation, and federal spending conditions.
Why Congress Cannot Force States to Enforce Federal Law
The controlling rule is the anti-commandeering doctrine. Congress cannot order state legislatures to pass laws, and it cannot order state officers to carry out federal programs. The Supreme Court has said so three times in ways that matter.
In New York v. United States (1992), the Court struck down a federal law that gave states a “choice” between regulating radioactive waste on federal terms or taking ownership of the waste themselves. Congress could not impose either option directly, so it could not impose the choice.1Legal Information Institute. New York v. United States
Five years later, Printz v. United States extended the rule to individual officers. The Brady Handgun Violence Prevention Act required local law enforcement to run background checks on gun buyers. The Court held that Congress cannot “conscript the State’s officers directly” to administer a federal regulatory program.2Legal Information Institute. Anti-Commandeering Doctrine The Court distinguished between requiring states to share information they already have and requiring them to actively investigate compliance with federal law. The second crosses the line.3Legal Information Institute. Printz v. United States
The most recent case, Murphy v. NCAA (2018), closed a loophole. The federal government argued that a law prohibiting states from authorizing sports gambling was not commandeering because it told states what they could not do, rather than what they had to do. The Court rejected the distinction. As it put it, “the basic principle — that Congress cannot issue direct orders to state legislatures — applies in either event.”4Supreme Court of the United States. Murphy v. National Collegiate Athletic Association
So the starting point is a floor, not a ceiling. States cannot be forced. Everything else in the enforcement picture is about what states may do voluntarily or what Congress may authorize them to do.
How States Actually End Up Enforcing Federal Law
Most enforcement activity that looks like a state enforcing federal law is really something else in the neighborhood. There are four common patterns.
Parallel State Statutes
Many states criminalize the same conduct federal law covers. Drug possession is the clearest example. When a state trooper arrests someone for possessing a controlled substance, the officer is enforcing the state’s own statute, even though the same conduct also violates federal law. No special authority is needed. The state has its own law on the books.
Joint Task Forces and Cross-Deputization
The FBI, DEA, and other federal agencies routinely partner with local police on investigations. In these arrangements, local officers can be formally cross-deputized, receiving temporary federal authority to enforce federal law directly. Federal drug enforcement statutes explicitly authorize this: the Attorney General can designate state and local officers to carry out DEA functions, including executing federal warrants and making arrests for federal felonies.5Office of the Law Revision Counsel. 21 USC 878 – Powers of Enforcement Personnel
287(g) Immigration Agreements
Immigration has its own formal cooperation mechanism. The Attorney General can enter written agreements with state or local governments allowing their officers to investigate, apprehend, and detain people suspected of violating federal immigration law. Officers operating under a 287(g) agreement must be trained in federal immigration law and work under federal supervision.6Office of the Law Revision Counsel. 8 USC 1357 – Powers of Immigration Officers and Employees Without an agreement, a local officer who encounters someone with an outstanding federal warrant can still detain the person, but only on general arrest authority, not as a deputized immigration agent.
Direct Federal Grants of Enforcement Power
In some areas Congress goes further and gives state officials the power to bring federal enforcement actions on their own. The clearest example is antitrust. Under the Clayton Act, any state attorney general can sue in federal court on behalf of the state’s residents for injuries caused by Sherman Act violations. These “parens patriae” actions let the state act as a legal representative of its citizens against anticompetitive conduct.7GovInfo. 15 USC 15c – Actions by State Attorneys General Congress has used the same model in various federal consumer protection statutes, giving state attorneys general concurrent authority to enforce alongside federal agencies and private plaintiffs.
Money as a Substitute for Command
Congress cannot order states to enforce federal law, but it can offer money and attach strings. The constitutional question is how tight those strings can be pulled before financial pressure becomes unconstitutional coercion.
The framework comes from South Dakota v. Dole (1987), which upheld a federal law that withheld a percentage of highway funds from any state that allowed people under 21 to buy alcohol.8Office of the Law Revision Counsel. 23 USC 158 – National Minimum Drinking Age The Court called the roughly 5% at stake “relatively mild encouragement” and set out a four-part test: spending conditions must serve the general welfare, be stated unambiguously, relate to the federal interest in the program, and not be so coercive that “pressure turns into compulsion.”9Justia Law. South Dakota v. Dole, 483 US 203 (1987)
The coercion prong sat dormant for 25 years. Then, in National Federation of Independent Business v. Sebelius (2012), the Court struck down the Affordable Care Act’s Medicaid expansion mechanism. The ACA threatened to cut off all existing Medicaid funding, not just the new expansion money, from states that refused to expand. Seven justices agreed this crossed the line into compulsion, because a state’s entire Medicaid budget is not a real choice.10Justia Law. National Federation of Independent Business v. Sebelius, 567 US 519 (2012) Modest incentives tied to a federal program remain fair game. Threatening a state’s existing funding stream to force compliance with a new program is not.
Where States Are Blocked from Acting
The other side of the enforcement question is when states try to enforce federal policy on their own initiative and get pushed back. Under the Supremacy Clause, federal law is “the supreme Law of the Land,” and state laws that conflict with it are invalid.11Constitution Annotated. Article VI – Clause 2 – Supreme Law Courts recognize three routes to preemption:
- Express preemption, where Congress states in the statute that federal law overrides state law on the topic. Medical device regulation is one example.
- Field preemption, where Congress has passed such a comprehensive scheme that it occupies the entire field and leaves no room for state rules. Alien registration has been held to be such a field.
- Conflict preemption, where a state law either makes compliance with both state and federal requirements impossible or stands as an obstacle to what Congress intended.12Congress.gov. Federal Preemption – A Legal Primer
The most instructive case is Arizona v. United States (2012). Arizona had passed a law making it a state crime to fail to carry federal alien registration documents, criminalizing unauthorized immigrants seeking employment, and authorizing state officers to arrest anyone they had probable cause to believe was removable from the country. The Court struck down all three provisions. The registration provision conflicted with the comprehensive federal registration scheme. The employment provision created a state crime where Congress had deliberately chosen not to criminalize the same conduct. The warrantless arrest provision gave state officers immigration enforcement powers that conflicted with the discretion Congress reserved for federal authorities.13Legal Information Institute. Arizona v. United States The requirement that state officers check immigration status during routine stops was not enjoined at that stage. The through-line: states can cooperate with federal immigration authorities, but cannot build parallel enforcement schemes in a field Congress has occupied.
The Right to Refuse
Because Congress cannot commandeer state resources, states and cities can also decline to help. That is where “sanctuary jurisdiction” policies come from. These policies typically instruct local officers not to honor federal immigration detainer requests, which ask local jails to hold someone past their normal release date so federal agents can pick them up. Detainers are requests, not orders. A local agency that ignores one is exercising a constitutional prerogative.
The federal government has repeatedly tried to pressure sanctuary jurisdictions by threatening to withhold federal grants, and courts have generally blocked those efforts. Under the Dole framework, funding conditions must relate to the purpose of the grant and be clearly stated by Congress. The executive branch cannot invent new conditions on money Congress has already appropriated. Federal courts blocked such attempts during the first Trump administration, and similar litigation has continued into 2025 and 2026.
Two Situations That Look Like Exceptions
Federal land trips people up. The Assimilative Crimes Act fills gaps in federal criminal law by borrowing state criminal statutes. If someone commits an act on federal land that Congress hasn’t specifically made a federal crime but that would be criminal under the law of the surrounding state, federal prosecutors can charge the person under the state statute as though it were federal law.14Office of the Law Revision Counsel. 18 USC 13 – Laws of States Adopted for Areas Within Federal Jurisdiction This is not states enforcing their own law; it converts state law into federal law for federal prosecution in federal court. On land under exclusive federal jurisdiction, state officers generally cannot even make warrantless arrests.15Congress.gov. Figure 2 – Potential Enforcement of State Law Where jurisdiction is concurrent, both federal and state officers can enforce their respective laws.
The National Guard is the other one. The Posse Comitatus Act generally prohibits using the federal military for civilian law enforcement, with violations punishable by up to two years in prison.16Office of the Law Revision Counsel. 18 USC 1385 – Use of Army, Navy, Marine Corps, Air Force, and Space Force as Posse Comitatus When Guard members serve under state command, which is their normal status, the Act does not apply, and they can participate in law enforcement if state law allows it. They fall under the Act’s restrictions only once federalized. A governor can deploy the state’s National Guard for law enforcement; the president generally cannot deploy federal troops for the same purpose without specific statutory authorization such as the Insurrection Act.