Hundreds of acres of public land near the Mexico border have been put under the control of the U.S. Navy for the sake of national security.

The Trump administration announced the plan, which is a continuation of a project started by the Theodore Roosevelt administration, on Dec. 10. The land is now designated as a “National Defense Area.”

Here’s what we know about the change. 

Militarized Zone

The 760 acres transferred to the military will now officially be a militarized zone.

The area runs from one mile west of the California-Arizona state line to the western edge of the Otay Mountain Wilderness Area in San Diego and Imperial counties.

The change is expected to assist with immigration enforcement, which is one part of the administration’s national security plan. The transfer allows the military to detain any illegal immigrants found in this area, and arrest anyone entering as if they were trespassing on a military base.

“This corridor is one of the highest-traffic regions for unlawful crossings along the southern border, creating significant national security challenges and contributing to environmental degradation,” the Interior Department said.

The military will be allowed to install temporary barriers and signage, and fence the area.

The department said the land was initially designated for “border protection purposes in 1907 by President Theodore Roosevelt.”

Southern California has been known to be rife with illegal border crossings. However, arrests for illegal crossings from Mexico have dropped to the lowest number since the 1960s. The Trump administration attributes that change to the crackdown by Border Patrol and other units sent to secure the southern border.

Secretary of the Interior Doug Burgum said in a statement on the recent handover that “President Trump has made it clear that securing our border and restoring American sovereignty are top national priorities.”

What Happens Now

According to the current administration, the Navy will step in to use the area to “strengthen operational capabilities while reducing ecological harm associated with sustained illegal activity.”

The Interior Department says it will coordinate with the Navy to ensure that the actions taken are “both lawful and targeted, reinforcing the historic role public lands have played in safeguarding national sovereignty.”

This isn’t the first such transfer. The federal government began handing control of portions of land along the border to the military in April of this year.

The first instance of this took place with a 170-mile swath of land along the border in New Mexico. This has also been done in Texas and Arizona.

In an April 11 memorandum, President Donald Trump said: “Through these enhanced authorities, U.S. Northern Command will ensure those who illegally trespass in the New Mexico National Defense Area are handed over to Customs and Border Protection or our other law enforcement partners,” Gen. Gregory Guillot, commander of U.S. Northern Command, said in a statement at the time.

Pushback

Legal experts assert that Trump’s use of troops flies in the face of the ban on laws being enforced by the military on U.S. soil and even puts the armed forces into a precarious position legally.

In September, a federal court ruled that the president’s use of the National Guard around Los Angeles violates the Posse Comitatus Act, the rule that bans the military from involvement in domestic law enforcement without an express authorization by Congress.

Rep. Sara Jacobs (D-Calif.) reacted to the news in a post on X, saying, “Expanding the militarized zone around San Diego doesn’t make us any safer. Ultimately, it’s expensive, useless, and attempts to normalize our troops being used to support domestic law enforcement which is anything but normal.”

Following the redesignation, several lawmakers have raised an alarm about the funds used to support border security, including Sens. Ron Wyden (D-Oregon) and Jeff Merkley (D-Oregon), who signed onto a letter to Secretary of War Pete Hegseth. 

“The investigation found that the Department of Defense (DoD) has obligated over $2 billion in support of Department of Homeland Security (DHS) and immigration enforcement in 2025, diverting it from the intended use—our national security and military servicemembers,” the lawmakers wrote.

“This is a baffling waste of military resources considering the appropriation of $170 billion to DHS to fund immigration enforcement earlier this year. We are particularly concerned that DHS may not be reimbursing DoD for these funds. Allowing DHS to continue to pick DoD’s pockets puts our military readiness at risk.“

Larger Border Mission

The president declared the southern border to be in a state of emergency through a presidential action the day he was inaugurated.

“America’s sovereignty is under attack.  Our southern border is overrun by cartels, criminal gangs, known terrorists, human traffickers, smugglers, unvetted military-age males from foreign adversaries, and illicit narcotics that harm Americans,” the proclamation said.

According to the president, the increased illegal immigration under the Biden administration constituted an “invasion” which caused “ widespread chaos and suffering in our country.”

The April 11 memorandum from the president’s office was one of several he issued, along with executive orders, that would secure the southern border.

The president has asked that Cabinet members take “all appropriate actions” to allow the Department of Defense, now called the Department of War, to have jurisdiction over federal lands that are deemed “reasonably necessary” to secure the border. 

“Our southern border is under attack from a variety of threats. The complexity of the current situation requires that our military take a more direct role in securing our southern border than in the recent past,” Trump wrote in the memo.

Over 7,000 troops have been deployed to areas of the border, along with helicopters, drones, and other military surveillance equipment.

Jacob Burg contributed to this report.