After the Department of Defense announced Wednesday that 1,500 troops would be initially assigned to the border mission, what bases and units they were coming from and what other forces could be included remained questions.

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.

Four military police units from Texas are among as many as 1,500 active-duty troops bound for the nation’s southern border to “bolster security,” military leaders said Friday. The movement, announced by the U.