On March 26, 2010 an explosion destroyed the South Korean warship Cheonan. This killed 40 men. Six men also disappeared. They are thought to be dead. An investigation into the sinking blamed North Korea. They said that a torpedo fired by a North Korean submarine had sunk the ship. North Korea said it had nothing to do with the Cheonan's sinking. They said they were not guilty. Lee said their denial was "laughable". He stopped all trade between the north and south. He also asked the United Nations to help them against North Korea. He also began sending radio messages to North Korean people.