How U.S. military presence is expanding at the southern border

KEY POINTS

Land was transferred to the Navy last month to create another "national defense area" along the southern border.

It is the fourth such designation establishing a U.S. military presence along a third of the U.S.-Mexico border.

While mostly illegal for the military to assist in law enforcement efforts, it sets up a defense against what Trump calls an "invasion."

Last month, the federal government expanded the military’s presence along the southern border after transferring control of 285 acres of public land in Arizona to the Navy. Now, more than a third of the state’s border is under the jurisdiction of the Marine Corps Air Station Yuma.

The new allotment of land is the fourth installation of what are being referred to as “national defense areas.” They are large tracts of Am