In broad daylight, masked assailants infiltrated North Korea’s embassy in Madrid, tied up the staff, stole computers and mobile phones, and fled the scene in two luxury vehicles. The group behind the late February operation is known as Cheollima Civil Defense, a secretive dissident organization committed to overthrowing the Kim dynasty, people familiar with the planning and execution of the mission told The Washington Post. The group’s alleged role in the attack has not previously been reported, and officials from the governments of North Korea, the United States and Spain declined to comment on it. The identity of the assailants...