A closer look at the two laws cited by Witt suggests it is far from evident that they authorize the diversion of funds to build a border wall. Section 2808 states that, if the president declares a “national emergency” that “requires the use of the armed forces,” he can use military construction funds to “undertake military construction projects, and may authorize the Secretaries of the military departments to undertake military construction projects, not otherwise authorized by law that are necessary to support such use of the armed forces.” It is far from clear whether any supposed emergency caused by undocumented immigration really “requires the use of the armed forces” or that a wall would be “necessary to support such use” of them. Indeed, as Ackerman points out, federal law actually forbids the use of the armed forces for domestic law enforcement within the United States (and immigration enforcement qualifies as such). Section 2293 also only applies to a declared war or emergency that “requires or may require use of the Armed Forces.” Even then, it only allows diversion of funds to build “authorized civil works, military construction, and civil defense projects that are essential to the national defense” (emphasis added). In this context, “authorized” likely means “authorized” by Congress, not just by the executive branch.

It is also worth noting that nothing remotely resembling a national security “emergency” is actually occurring at the southern border, and that a border wall would do virtually nothing to protect the US against any kind of terrorism or security risk. It may well not even do much to reduce undocumented immigration.