A tiny Pentagon office had spent months investigating conspiracy theories about secret Washington UFO programs when it uncovered a shocking truth: At least one of those theories had been fueled by the Pentagon itself. The congressionally ordered probe took investigators back to the 1980s, when an Air Force colonel visited a bar near Area 51, a top-secret site in the Nevada desert. He gave the owner photos of what might be flying saucers. The photos went up on the walls, and into the local lore went the idea that the U.S. military was secretly testing recovered alien technology. But the...