In March, a South Korean warship was torpedoed, killing 46 sailors and sinking the vessel. Recent evidence strongly implicates North Korea as the most likely power responsible for the attack, though Pyonyang denies any involvement. Now, South Korean President Lee Myung-bak has said his country will boost its defense, sever all trade with North Korea and deny North Korean merchant ships access to their sea lanes. The U.S. has backed the South Korean stance. 

But this is not the first time North Korea has taken a hostile maritime policy, nor is this the most explicit act of aggression by Pyongyang.