Justice Department expands military's power to detain migrants near border

The U.S. Justice Department has issued a legal opinion that significantly broadens the U.S. military's powers to detain migrants suspected of trespassing on Department of Defense-controlled land near the southern border. This move is part of the Trump administration's intensified efforts to deter illegal immigration.

The opinion from the Justice Department's Office of Legal Counsel specifically addresses large tracts of land designated as "National Defense Areas" (NDAs) along the U.S.-Mexico border. While the Posse Comitatus Act generally prohibits the military from enforcing domestic laws, these NDAs allow soldiers to detain migrants accused of entering restricted zones closed to civilians. This has enabled the Justice Department to prosecute those apprehended for trespassing on military property.

Under the new opinion, military personnel are authorized to arrest individuals who attempt to leave an NDA before they can be apprehended for trespassing. Deputy Assistant Attorney General William Hyde stated that arresting trespassers just outside an NDA would not violate the Posse Comitatus Act due to express statutory authorization and the military commander's traditional protective powers. The administration has already established six NDAs in Arizona, California, New Mexico, and Texas, and has indicated the possibility of designating additional zones.

Signs in both English and Spanish have been posted in some areas to warn against unauthorized entry. U.S. Northern Command has stated that service members can apprehend trespassers and transfer them to law enforcement. Since April 2025, the Justice Department has charged hundreds of individuals with trespassing on national defense areas, typically for violating military security regulations.

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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.