DOJ Expands Military Authority To Detain Migrants Near U.S.–Mexico Border

The Justice Department's Office of Legal Counsel (OLC) on Friday issued a written opinion broadening the U.S. military's authority to detain migrants accused of trespassing on land the Department of Defense has designated as National Defense Areas (NDAs) along the southern border.

The opinion covers wide stretches of territory in Arizona, California, New Mexico and Texas that the Trump administration in its second term has declared NDAs to discourage illegal crossings. While the Posse Comitatus Act normally limits the military's role in domestic law enforcement, the OLC concluded that, under the circumstances described, military personnel may arrest individuals who leave an NDA before civilian authorities can take custody.

"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 in the opinion.

The opinion notes that six NDAs have been established so far and that the Defense Department has indicated it may designate additional zones. Because some NDAs are long, linear strips of land that do not resemble conventional military bases, the Pentagon has posted signs in English and Spanish warning that the areas are restricted and that unauthorized entry is prohibited.

U.S. Northern Command says service members may apprehend people who trespass onto NDAs and transfer them to civilian law-enforcement authorities. Prosecutors began filing charges for incidents in April 2025 and have since brought hundreds of cases. The most common counts tied to entering an NDA are violations of military security regulations and unlawful entry onto military, naval or Coast Guard property—both misdemeanors—though other immigration-related charges can also apply.

The Pentagon made the U.S.–Mexico border a top mission in the opening days of the president's second term and announced plans to increase troop deployments. The Defense Department reports that more than 20,000 service members have served on border deployments since that effort began.

The OLC opinion is advisory and does not create binding legal precedent; courts could still rule differently if the policy is challenged. The guidance arrives alongside other administration measures that have tightened asylum access for people entering irregularly, a suite of policies officials say has coincided with a sharp drop in Border Patrol apprehensions.

According to agency data, Border Patrol recorded fewer than 240,000 migrant apprehensions in fiscal year 2025, down from roughly 1.5 million in FY2024 and about 2 million in FY2023. Apprehensions fell further in FY2026, with about 81,000 detentions recorded with two months remaining in the fiscal year.

Bottom line: The OLC opinion expands the situations in which military personnel may detain individuals near NDAs, but because it is advisory, the policy could face legal challenges and political scrutiny over the proper role of the armed forces in domestic immigration enforcement.