Kim Jong-un called it a “vicious cancer” corrupting young North Koreans’ “attire, hairstyles, speeches, behaviors.” His state media has warned that if left unchecked, it would make North Korea “crumble like a damp wall.” After winning fans around the world, South Korean pop culture has entered the final frontier: North Korea, where its growing influence has prompted the leader of the totalitarian state to declare a new culture war to stop it. But even a dictator may have trouble holding back the tide. In recent months, hardly a day has gone by without Mr. Kim or state media railing against “anti-socialist and nonsocialist” influences spreading in his country, especially South Korean movies, K-dramas and K-pop videos. As part of a panicked attempt to reassert control, Mr. Kim has ordered his government to stamp out the cultural invasion.