A Marine adjusts concertina wire along the southern border wall near San Ysidro, Calif., Jan. 27, 2025. (Caleb Goodwin/U.S. Marine Corps)

Marines on Monday began to hang temporary barriers along a wall in San Ysidro, Calif., marking the U.S. border with Mexico as an Air Force flight crew prepared to take off from Fort Bliss, Texas, to Ecuador with the military’s fourth planeload of deported migrants.

The efforts are part of the military’s fulfillment of an executive order signed just more than a week ago by President Donald Trump to make border security a top priority of the Defense Department.

The 500 Marines in California were the first active-duty troops deployed for the mission, which has climbed from 1,500 to 1,600 in the past week, according to a spokesperson for U.S. Northern Command, the military combatant command leading the mission.

Soldiers prepare to depart Jan. 25, 2025, at Hunter Army Airfield, Ga., for a mission along the southern border. (Department of Defense)

The remaining troops — pulled from Army units across the country — have first stopped in El Paso, Texas, and San Diego to begin learning about the mission before moving to various locations in the southwest, NORTHCOM said Tuesday.

About 750 troops had arrived at Fort Bliss, an Army post in El Paso, as of Tuesday, according to base officials. The base is often used by the Army to prepare and deploy service members and is supporting the staging of equipment and materials that will go with soldiers to the border.

Those soldiers and Marines will carry their service weapons with them for self-defense, NORTHCOM said. Photos released Monday showed soldiers arriving from bases including Fort Campbell, Ky., and Joint Base Lewis-McChord, Wash., dressed in full combat gear and helmets and carrying M-4 rifles.

“The decision to arm military members is based on particular security situations. As of right now, none of the forces moving to the border are intended to be used for law enforcement,” according to NORTHCOM.

Customs and Border Protection will choose where troops are sent along the 1,950 miles of border that covers four states, the command said. They join about 2,500 National Guard troops already deployed to the border and will join a mission that began in 2018 during Trump’s first term as president.

An Air Force loadmaster directs a forklift driver loading Army equipment onto a C-17 Globemaster III at Fort Carson, Colo., Jan. 26, 2025. (Department of Defense)

Filling a support role for Customs and Border Protection, troops conduct vehicle maintenance, light construction, transportation, and observation and detection from the ground and air.

Mark Cancian, a senior adviser with the Washington-based think tank Center for Strategic and International Studies, wrote Monday that these actions have become common in the past 30 years. However, if the mission scales to higher troop levels — such as the 10,000 that the Pentagon confirmed defense officials are discussing — that would be unprecedented, he said.

“President Trump sent troops to the border to emphasize its importance. Other presidents have done the same. However, it would be a mistake for the military to take over this mission. Stopping criminals is a law enforcement activity for which the military is poorly suited. Military personnel are trained and equipped to destroy enemy forces,” Cancian said.

For the deportation flights, the Air Force conducted three last week to Guatemala, and NORTHCOM said the flights will continue as needed. The Coast Guard has flown those people selected for deportation from locations across the U.S. to Texas and California, where they are then flown out of the country by the Air Force. The Department of Homeland Security provides in-flight security on the aircraft, and the State Department coordinates with the country receiving the flight, the military said.

All these efforts are separate from the state-sponsored mission of the Texas National Guard, which Monday announced an additional 400 troops to supplement its work in support of state police to deter crime at the border. It has roughly 5,000 troops deployed within the state.

Texas began its border mission in 2021, and Texas Gov. Greg Abbott requested last week for the federal government to reimburse the state for the $11 billion that it has spent. Sen. John Cornyn, R-Texas, has pledged his support for this.