Troops have assisted at the border before and will be limited to support roles unless the administration invokes the Insurrection Act.

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.

More than 400 additional Texas National Guard troops arrived at the Texas-Mexico border Monday. Governor Greg Abbott deployed the troops to the Rio Grande Valley, where he said they'll work side-by-side with U.S. Border Patrol agents to keep migrants from illegally entering the country.