Active-duty military troops are set to begin arriving in El Paso, Texas, and San Diego on Thursday evening, marking the initial phase of deploying new forces to secure the southern border, according to defense officials.

About 500 Camp Pendleton-based Marines and sailors are being deployed to the U.S.-Mexico border as part of President Trump’s mandate to secure the southern border.

The I Marine Expeditionary Force is made up of about 53,000 active-duty troops, who are primarily stationed at Camp Pendleton, as well as at Marine Corps Air Stations Miramar and Yuma, and the Marine Corps Air/Ground Combat Center at 29 Palms, California.