There are already about 2,500 U.S. National Guard and Reserve forces there. Zoom in: The Pentagon will also provide military aircraft to support the Department of Homeland Security's deportation flights of more than 5,000 undocumented immigrants from the areas of San Diego, California; and El Paso, Texas.

The active duty forces will join the roughly 2,500 US National Guard and Reserve forces already there. There are currently no active duty troops working along the roughly 2,000-mile border.

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