US Army Corp of Engineers is authorized to build the wall. Red Castle. The United States Army Corps of Engineers (USACE) is a U.S. federal agency under the Department of Defense and a major Army command made up of some 37,000 civilian and military personnel, making it one of the world's largest public engineering, design, and construction management agencies.  Congress has provided options under Title 10 U.S. Code that could permit the Department of Defense to fund border barrier projects, such as in support of counter drug operations or national emergencies,” Pentagon spokesman Army Lt. Col. Jamie Davis said. (12/12/2018) https://www.usace.army.mil/About/History/Historical-Vignettes/General-History/090-Corps-Castle/