In a show of force, the aircraft carrier USS George Washington arrived in South Korea Wednesday. It's expected to take part in military exercises next week.
Relations with the north have gone straight downhill since a South Korean warship was sunk in March, an attack blamed on North Korea.
The 46 sailors who died never had a chance, CBS News National Security Correspondent David Martin reports.
The sinking of the Cheonan, which was ripped in two last March off the west coast of the Korean peninsula, was likely similar to that of an out-of-service Australian warship that was used for target practice in 1999.
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The 500-pound warhead detonated directly below the Australian ship, lifting it out of the water, sending its superstructure overboard, bending the steel girders that make up its spine. A second later, the ship crashed back into the water, breaking its spine, splitting it in two and sending it to the bottom.
Secretary of State Hillary Clinton traveled to the demilitarized zone Wednesday between North Korea and South Korea with Defense Secretary Robert Gates to announce plans to impose new sanctions against the north.
"To date, we have seen nothing that gives us any reason to believe that North Korea is ready to end its provocative, belligerent behaviors," Clinton said.
The sinking of the Cheonan took place under of darkness. When investigators raised the severed hull and brought it back to port, readings from seismic stations, testimony of surviving sailors and analysis of the bent and twisted metal all led to the conclusion the Cheonan had been sunk by a torpedo with a 500-pound warhead.
There was no proof of who fired the shot. Then South Korean fishermen dredged up the remnants of a torpedo from the ocean bottom. The corrosion on the metal parts indicated it had been underwater from about the same time the Cheonan was sunk, and its design exactly matched that of a North Korean torpedo.
It was an act of war although the North Koreans deny they did it, but the only real mystery about the sinking of the Cheonan is why they did it.
More on Warship Sinking
N. Korean, U.N. Officials Discuss Sunken Ship
UN Condemns Sinking of S. Korean Ship
Obama Criticizes N. Korea for Sinking Warship
Korean Tensions over Ship Sinking Worry U.N.
S. Korea: North to "Pay Price" for Sinking Ship
Clinton Convinced North Korea Sunk South's Ship