SEOUL, South Korea — North Korea on Thursday threatened unspecified retaliation after the U.S. flew long-range bombers over South Korea during training with South Korean warplanes, which North Korea views as practice for an attack against it. The U.S. flew the B-1B bombers Tuesday to train with other U.S. and South Korean fighter jets. South Korea’s Defense Ministry had said the drill was meant to show the two countries’ combined deterrence capability against North Korea’s advancing nuclear