ON FEB. 1 LAST YEAR, Montana residents gawked upwards at a large white object hovering in the sky that looked to be another moon. The airborne object was in fact a Chinese spy balloon loaded with cameras, sensors, and other high-tech surveillance equipment, and it set off a nationwide panic as it drifted across the midwestern and southern United States. How much information the balloon gathered—if any—remains unknown, but the threat was deemed serious enough that an F-22 U.S. Air Force jet fired...