Legal Background: Distinguishing Between Law Enforcement Powers

Though all U.S. law enforcement actors share a common goal of keeping Americans safe, the law differentiates between who can enforce certain laws and how they can do so. This backgrounder outlines aspects of the legal authorities governing three government actors—local and state police, federal immigration agents, and the military.

This backgrounder is part of a CNAS series on distinguishing between law enforcement and military appearance—an issue intimately linked to each actor’s underlying legal authority. Distinguishing visually between these actors matters because each actor operates with a specific legal mandate—as well as legal constraints. If citizens cannot tell who is making the arrest, they cannot effectively keep law enforcers within their legal bounds.

State and Local Law Enforcement Powers

Under the 10th Amendment, which commands “the powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people,” states are the default law enforcers. Unlike federal agents, who are limited to those powers specifically “delegated” to them by the constitutional text, state officers may act in any way that furthers the public good. They are free, in other words, to exercise “police powers.”

Justice John Marshall coined the term “police power” in 1827 to describe powers “which unquestionably remain and ought to remain with the states.” In 1991, the Supreme Court in Barnes v. Glen Theatre, Inc. reiterated a broad view of police powers that encapsulates any state action that furthers “public health, safety, and morals.” In 2000, the Supreme Court in United States v. Morrison stressed that there is “no better example of the police power . . . than the suppression of violent crime and vindication of its victims.” And in 2012, the Supreme Court in National Federation of Independent Business v. Sebelius described police powers as including “vital functions of modern government” like “punishing street crime [and] running public schools.”

In the law enforcement context, police powers can look like routine traffic stops, emergency responses, or criminal investigations carried out by state troopers, local sheriffs, or any general police officers. As long as those officers comply with the Supremacy Clause (which bars state law enforcement practices that conflict with federal law), the Bill of Rights, and all other constitutional protections, state actors are free to enforce both state and federal law.

State or local police also can conduct immigration enforcement, entering into legal agreements that essentially deputize local police with the power of federal immigration agents like Immigration and Customs Enforcement (ICE) or Customs and Border Protection (CBP) agents. State and local governments across the country have chosen to sign such agreements, though under the anticommandeering doctrine, they cannot be forced to do so. As Printz v. United States reiterated, the federal government cannot “compel the States to implement, by legislation or executive action, federal regulatory programs.” Whether the police become an arm of federal immigration enforcement is a choice only a state or local government can make.

Federal Immigration Authorities: ICE and CBP

Though federal agencies like ICE and CBP cannot exercise a general police power, they do have broad power to enforce federal immigration laws. Though recent events show how agents might go beyond that remit, federal law nonetheless lays out the distinct bounds of ICE’s and CBP’s immigration enforcement powers.

ICE and CBP agents are both “immigration officers” under the Immigration and Nationality Act (INA) of 1952, the federal government’s flagship immigration statute. Under the INA, immigration officers can interrogate aliens without a warrant, conduct warrantless searches “of any person seeking admission to the United States,” execute search warrants, investigate by “administer[ing] oaths and … tak[ing] and consider[ing] evidence,” and under 8 CFR § 287.7, issue detainers (essentially requests for local law enforcement to hand over people detained for immigration violations). Immigration officers also can make warrantless arrests for immigration violations and for felonies related to admission or removal.

Additionally, all CBP agents and many ICE agents have a general arresting power, though this power is more qualified compared to state or local police.

Under 8 CFR § 287.5—a regulation implementing the INA—all CBP agents and many ICE agents (specifically deportation officers), provided both are properly trained, can arrest anyone who commits any federal crime in their presence. The same group also can arrest anyone for a federal felony if the officer (1) reasonably believes a felony will or has occurred, (2) is enforcing immigration law at the time of arrest, (3) believes the arrestee will likely escape before a warrant is issued, and (4) is sufficiently trained. Notably, these powers apply even when an arrestee’s citizenship is not in question.

When it comes to geography, the powers of ICE and CBP agents diverge, with CBP agents alone enjoying unique, geographically limited “patrolling” powers.

Under this “patrolling” power, CBP agents can “access [] private lands, but not dwellings, for the purpose of patrolling the border” (i.e., to set up checkpoints), but only within 25 miles of an international border. In addition, without a warrant they can board “vessels, railway cars, aircraft, conveyances, and vehicles to search for aliens,” but only within a “reasonable distance” of an international border.

The Department of Homeland Security (DHS) defines a reasonable distance to mean “within 100 air miles from any external boundary of the United States,” and it has interpreted this definition liberally. Nearly two-thirds of the U.S. population falls within CBP’s own definition, including all of Chicago. (CBP’s definition of international maritime borders means Chicago falls within 100 miles of Canada). Within this 100-mile range, DHS does not need a warrant to enter or search “vessels, railway cars, aircraft, conveyances, and vehicles,” but DHS must still “develop particularly probable cause to conduct a lawful search.” While at the physical border or its functional equivalent, however, CBP agents enjoy a powerful “border search exception.” Under United States v. Ramsey, courts assume that searches near the border are reasonable, and Fourth Amendment requirements are significantly easier to meet.

However, CBP agents can and do venture outside the 100-mile range, though they must relinquish their unique “patrolling” powers when they do so. Likewise, ICE agents are free to operate anywhere within the 100-mile range, though they can never gain “patrolling” powers. Additionally, since both CBP and ICE agents are “immigration officers” under the INA, they never relinquish core statutory powers like the authority to interrogate suspected aliens or make arrests.

Ultimately, since CBP and ICE agents can work in the same jurisdiction as state and local police and, subject to a few constraints, can arrest anyone who commits a crime in their presence, it is unclear if—or when—federal immigration efforts unconstitutionally infringe on state police powers.

The states of Illinois and Minnesota believe ICE agents have already crossed the line, with Illinois arguing in a recent suit that ICE is using federal immigration authority as a backdoor means to “displace” traditional state law enforcement—a violation of the 10th Amendment.

Likewise, the state of Minnesota argues that even if federal agents are not directly supplanting Minnesota’s police powers, they are unconstitutionally burdening them by “overwhelming” local police and essentially forcing local schools to cancel classes.

The government has moved to dismiss both the Minnesota and Illinois cases. If either case proceeds, it could clarify how federal immigration enforcement powers interact with state police powers.

Limits on Military Engagement in Law Enforcement

Under the Posse Comitatus Act, the military is generally barred from conducting law enforcement activities. However, there are a few exceptions (including a general carveout for the Coast Guard) that could allow U.S. soldiers to enforce federal law or, at least, assist federal agents in carrying out the law.

First, the president could federalize the National Guard under the Insurrection Act, which would exempt the military from Posse Comitatus restrictions. The last time the Insurrection Act was invoked was during the 1992 Los Angeles riots, and no president has ever used the act to enforce immigration laws.

Second, under 10 U.S.C. § 12406, the president could “call forth” the National Guard to meet an emergency and protect federal agents. President Donald Trump did this in June 2025 when he ordered the Guard to “temporarily protect ICE and other United States Government personnel” from the unrest in Los Angeles. Though the Posse Comitatus Act still applies under Section 12406, constraining the Guard to the role of a passive assistant or protector, the lines between assisting and enforcing can blur, and it can be unclear exactly what functions the Guard is empowered to carry out. Additionally, in Trump v. Illinois, the Supreme Court significantly limited the president’s ability to use Section 12406. That case has since been dismissed, though the Supreme Court’s ruling remains on the books.

Third, the president could take advantage of the National Guard’s “hybrid” status by empowering the state-controlled Guard with the authority of federal agents like ICE or CBP officers. As Professor Mark Nevitt writes, hybrid status provides a situation where the Guard “performs federal missions using federal funds while remaining under state command and control.”

Under this hybrid status, cooperating states could enter into legal agreements where the DHS secretary designates Guard soldiers as “immigration officers” (as allowed under 8 CFR § 2.1) and puts them through the required training (as defined in 8 CFR § 287.1). This is the kind of deal struck in the final memorandum of agreement between Texas and DHS in early 2025, and at least 20 other Republican-led states made lesser agreements where the Guard can provide passive assistance like “case management, transportation and logistical support, and clerical support.”

Finally, as Professor Nevitt notes, 32 U.S.C. § 502—particularly its broad language empowering the secretaries of the Army or Air Force to demand the Guard do “other duties”—could be used as a backend means of enforcing immigration laws. Like the Insurrection Act, however, no one has used Section 502 in this way.

Conclusion

Even if they can become blurred in practice, the lines between general law enforcement, federal immigration enforcement, and military “assistance” are legally distinct. No matter what they wear, or what powers they claim, each actor has specific laws constraining their behavior.

About the Author

Matthew Kroscher is a second-year law student at George Mason University’s Antonin Scalia Law School and was a spring 2026 CNAS legal intern.

About the Series

Building on its prior work on issues at the intersection of federalism, national security, domestic deployment, and law enforcement activities, the CNAS National Security Law Program is publishing a commentary series that assesses the legal and policy considerations for improving the framework governing law enforcement use of military uniforms and gear. Read more about the series here.

The CNAS project on federalism and national security was initiated with support from the Democracy Innovation Fund, Defending Democracy Together Institute. Continued work on domestic deployment and related issues has been made possible by support from the William and Flora Hewlett Foundation.

About the Center for a New American Security

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