South Korea, U.S. begin exercises as North Korea threatens attack

South Korean and U.S. troops began large-scale military exercises on Monday in an annual test of their defenses against North Korea, which called the drills "nuclear war moves" and threatened to respond with an all-out offensive. South Korea said the exercises would be the largest ever following North Korea's fourth nuclear test in January and a long-range rocket launch last month that triggered a U.N. Security Council resolution and tough new sanctions. Isolated North Korea has rejected critici