The Pentagon will establish two new National Defense Areas—one spanning roughly 250 miles of the Rio Grande in southern Texas and another along a strip of Arizona bordering Yuma—U.S. officials said. The tracts will be administered as extensions of Joint Base San Antonio and Marine Corps Air Station Yuma, respectively, empowering troops inside the zones to temporarily detain migrants and other trespassers until U.S. Border Patrol agents arrive. With the latest designations, four militarized buffer zones now run along the Southwest frontier, placing nearly one-third of the 1,951-mile border under military jurisdiction. About 11,900 service members are deployed to the region under President Donald Trump’s 2025 emergency order, which sidesteps Posse Comitatus restrictions by treating the areas as federal defense property. Defense officials say the expansion is intended to close gaps used by smugglers and has coincided with a sharp drop in crossings: Border Patrol arrests fell 30 percent in June, reaching a six-decade low. Critics—including civil-rights groups, outdoor enthusiasts and some local governments—argue the policy blocks public access to federal land and criminalises migrants; more than 1,400 people have already been charged with trespassing in the zones, and court challenges are pending in New Mexico and Texas.