To do more than deploy the National Guard to patrol the US border with Mexico, the President will most likely need Congressional approval. See: TITLE 18 > PART I > CHAPTER 67 > § 1385 entitled Use of Army and Air Force as posse comitatus. The Posse Comitatus Act - Prohibits search, seizure, or arrest powers to US military personnel. Amended in 1981 under Public Law 97-86 to permit increased Department of Defense support of drug interdiction and other law enforcement activities. Certainly preventing illegal border crossings would involve "other law enforcement activities", and perhaps whatever activities have been...