North Korea is responding to allegations that it destroyed a South Korean warship in March by shredding an agreement between the two countries designed to prevent accidental skirmishes at sea. The announcement comes two days after North Korea said it would cut all ties with the South. An international investigation concluded a North Korean torpedo was behind the attack on the Cheonan, which killed 46, but the secretive dictatorship denies any involvement and is threatening war if South Korea enters its waters. South Korea is conducting military exercises at sea, a show of force that's adding to the current tension.