Just Security

04/29/2025

On April 11, President Donald Trump announced his intention to turn a strip of federal land along the U.S. southern border into a massive “National Defense Area,” effectively creating a new 170-square-mile U.S. military installation. The plan was described in a National Security Presidential Memorandum (NSPM-4), titled “Military Mission for Sealing the Southern Border of the United States and Repelling Invasions.” The document sets in motion the creation of a military area that is twice the size of Washington, D.C. It also expands the U.S. military’s role in stopping migrants crossing the border.

A Transfer of Federal Lands to the Department of Defense

First, Trump is directing the transfer of a 60-foot strip of federal land lying parallel to the U.S.-Mexico border from control of three federal agencies (Interior, Agriculture, and Homeland Security) to the Department of Defense. This land, known as the Roosevelt Reservation, is a noncontiguous strip of federal land that snakes along the southern border from New Mexico to California. President Theodore Roosevelt established the reservation in 1907 to prevent smuggling between the United States and Mexico. In initiating this massive land transfer, Trump highlighted the need for DOD jurisdiction over these lands so that the military could conduct activities such as border-barrier construction and the placement of detection and monitoring equipment.

Bypassing Posse Comitatus Act Restrictions

Second, the establishment of a National Defense Area increases the military’s role at the southern border, effectively bypassing longstanding legal restrictions put in place by the Posse Comitatus Act. The Posse Comitatus Act prohibits federal military forces from being used in a law enforcement capacity, such as searching, seizing, detaining, and arresting people entering the country. The Posse Comitatus Act does not apply to National Guard members under state control. Today, there are approximately 10,000 active-duty soldiers operating at the border alongside members of the National Guard. By statute, Congress has granted the military broad authority to protect federal property. Now, federal military forces at the southern border will be empowered to arrest, detain, search, and seize anyone—migrant or otherwise—that attempts to cross into the new National Defense Area.

Muddying the Waters of the Military Purpose Doctrine

Third, NSPM-4 continues the trend of using language that ties immigration flows at the border with broader national security authorities and core military missions. For example, NSPM-4 exclaims that the southern border is “under attack from a variety of threats.” Trump is therefore assigning the military the “mission of repelling the invasion and sealing the United States southern border.” Trump has invoked similar language in invoking the Alien Enemies Act, declaring that the United States is being invaded by a predatory incursion of a transnational gang.