Can Trump build the wall without Congressional approval?

Border Field State Park/Imperial Beach, San Diego, California. The fence between the USA and Mexico along the Pacific Ocean just south of San Diego. Photo credit: Tony Webster/CC 2.0

It’s day 17 of the government shutdown. The sticking point is still the billions of dollars for the border wall. President Trump says he’s found a way around Democrats’ intransigence by declaring a national emergency along the southern border. He says that would allow him to divert money from the military to fund the wall, and he’d use members of the armed services to build it. That might not be legal.