Orange no-entry signs posted by the U.S. military in English and Spanish dot the New Mexico desert, where a border wall cuts past onion fields and parched ranches with tufts of tall grass growing amidst wiry brush and yucca trees.

The Pentagon is establishing a fourth military defense zone along the U.S.-Mexico border, where American soldiers can apprehend noncitizens on charges of trespassing, Chief Pentagon spokesman Sean Parnell said Wednesday.