See the military zone in the Roosevelt Reservation, where US troops can arrest migrants along the US-Mexico border

Earlier this month, President Donald Trump signed an executive order giving the Department of Defense jurisdiction over the Roosevelt Reservation, a 60-foot-wide stretch of federal land that runs through New Mexico, Arizona, and California.

The Department of the Interior previously maintained the nearly 110,000-acre swath of federal land before the "emergency withdrawal and transfer of administrative jurisdiction" on April 15. The order excludes the jurisdiction of "Federal Indian Reservations."

The Army requested the jurisdiction transfer to increase patrols by federal personnel and implement security measures and infrastructure to "curb illegal cross-border activities," like illegal immigration and drug and human trafficking, according to a DOI statement.

The April 11 order outlines a phased rollout, starting by testing the implementation of a controlled perimeter in a "limited sector of federal lands" in New Mexico.

For the next 45 days, soldiers will detect and patrol routes and apprehend trespassers in the area. They will also construct temporary barriers and install signs to indicate the boundaries of the military zone.

Following the initial assessment and the Army's acceptance of jurisdiction, the federal land will become a "national defense area," granting the Pentagon the authority to establish and enforce security measures around it.

The 170-square-mile stretch of land will be considered an extension of Fort Huachuca in Arizona — despite not being physically connected to all of it — and remain under control by the Army for three years, according to the DOI press release.

"Our southern border is under attack from a variety of threats," Trump said in the memo. "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."

While under the Army's control, the militarized zone, known as the "New Mexico National Defense Area," will be governed and defended like any other military base, allowing US troops to search and temporarily detain trespassers before appropriate civilian or federal law enforcement officials take over.

Military personnel will also work alongside US Customs and Border Patrol agents at the southern border "to repel unlawful mass migration, narcotics trafficking, migrant smuggling, human trafficking, and other cross-border criminal activities," per a Pentagon press release.

Though thousands of military personnel have already been deployed to the southern border, the Posse Comitatus Act bars active-duty troops and federalized National Guardsmen from acting as domestic law enforcement.

The US military could be authorized to enforce civil or criminal law on US soil through the Insurrection Act of 1807, which provides a statutory exception to the PCA in specific circumstances of civil disorder or armed rebellion. To enact it to stop migrants, however, would be an extraordinary use of the federal law.

Since Border Patrol agents and civilian law enforcement are only allowed to apprehend migrants, the vast majority of troops deployed to the border have been tasked with "enhanced detection and monitoring," Gen. Gregory Guillot, the commander of the US Northern Command, said.

Some personnel "operating in close proximity to the border" have been armed with rifles or pistols, Guillot said. Military officials have sought authorization for troops to shoot down suspected cartel drones surveilling the border.

Thousands of active-duty troops aren't the only military assets the Trump administration is using to ramp up its crackdown.

Two US Navy warships, several military aircraft, and over 100 combat vehicles have been deployed to support the Pentagon's southern border operation.

On April 1, Defense Department officials estimated the cost of militarization at the US-Mexico border since January 20 at roughly $376 million, or about $5.3 million per day.

Traditionally seen as a steward protecting public federal lands, the DOI emphasized the "environmental crisis" at the border — not caused by the accelerated military build-up or deferred mitigation efforts, but by the migrants themselves.

The department said "repeated foot traffic, unregulated vehicle use, and the creation of informal trails or camps" cause harm to "ecologically sensitive areas along the southern border."

"Securing our border and protecting our nation's resources go hand in hand," Secretary of the Interior Doug Burgum said during a visit to New Mexico. "This transfer reflects Interior's commitment to public safety, national security, and responsible stewardship of our public lands."