>>4611217 (Q)
Posse Comitatus Act and the US Military on the Border
www.thoughtco.com/posse-comitatus-act-military-on-border-3321286
In addition, the troops will assist with "counternarcotics enforcement" duties until additional Border Patrol agents are trained and in place. The Guard troops may also assist in the construction of roads, fences, surveillance towers and vehicle barriers necessary to prevent illegal border crossings.
Under the Defense Authorization Act for FY2007 (H.R. 5122), the Secretary of Defense, upon a request from the Secretary of Homeland Security, can also assist in preventing terrorists, drug traffickers, and illegal aliens from entering the United States.