The Justice Department issued a legal opinion Friday expanding the U.S. military’s authority to detain migrants accused of trespassing in Defense Department-controlled areas near the southern border, a move tied to President Trump’s intensified immigration enforcement campaign.
The opinion, issued by the Justice Department’s Office of Legal Counsel, focuses on broad stretches of land along the U.S.-Mexico border that the second Trump administration has labeled “National Defense Areas” as part of its effort to discourage illegal crossings.
Federal law generally bars U.S. troops from carrying out domestic law enforcement under the Posse Comitatus Act. But the creation of these restricted zones has given service members a role in detaining migrants accused of entering areas closed to civilians. Those arrests have, in turn, enabled the Justice Department to bring trespassing cases involving military-controlled land.
In Friday’s opinion, the department concluded that military personnel may arrest people even after they have stepped outside a national defense area, as long as they are being pursued for alleged trespassing inside the restricted zone.
“In sum, we conclude that the use of military personnel to arrest trespassers just outside of an NDA would not violate the [Posse Comitatus Act], given the express statutory authorization and the military-purpose of a commander’s traditional protective power,” Deputy Assistant Attorney General William Hyde wrote.
The Trump administration has established six national defense areas along the southern border so far, spanning parts of Arizona, California, New Mexico and Texas. The Justice Department opinion also noted that the military has indicated more such areas could be designated later.
Government Accountability Office
Because the zones cover large strips of land that may not look like traditional military installations, the Defense Department has posted warning signs in some locations in both English and Spanish, notifying the public that entry is restricted and unauthorized access is forbidden.
U.S. Northern Command, which oversees American military operations across North America, says on its website that service members may apprehend individuals who enter national defense areas without authorization and hand them over to the appropriate law enforcement agencies.
The Justice Department began charging individuals who were apprehended in April 2025 and since then has charged hundreds more. The specific charges for trespassing a national defense area are most commonly violating a military security regulation and entering military, naval or Coast Guard property, both of which are misdemeanors. There can also be other charges associated with crossing the border that aren’t directly related to trespassing onto military land.
The Pentagon made the U.S.-Mexico border its first mission during the initial days of President Trump’s second term by announcing plans to ramp up U.S. forces there. Since then, the Defense Department says more than 20,000 service members have served deployments on the border.
The OLC’s legal opinion is advisory only and does not set any legal precedent.
In addition to enlisting the military’s help at the southern border, the Trump administration has virtually closed the asylum system for those entering the U.S. illegally. The crackdown has coincided with a dramatic drop in illegal crossings there.
In fiscal year 2025, Border Patrol reported fewer than 240,000 migrant apprehensions along the southern border, down from 1.5 million and 2 million in fiscal years 2024 and 2023, respectively, according to agency data. The number has fallen even more in fiscal year 2026, with Border Patrol recording 81,000 apprehensions, with two months left.
