There are four important points that make it clear that a North Korean submarine did not sink the South Korean corvette.
Fact 1.North Korean submarines are not stealthy enough to penetrate heavily guarded South Korean waters at night and remain undetected by the highly touted anti-submarine warfare units of the American and South Korean forces. A North Korean submarine would be unable to outmaneuver an awesome array of high-tech Aegis warships, identify the corvette Cheonan and then slice it in two with a torpedo before escaping unscathed, leaving no trace of its identity.
Fact 2. The sinking took place not in North Korean waters but well inside tightly guarded South Korean waters, where a slow-moving North Korean submarine would have great difficulty operating covertly and safely, unless it was equipped with AIP (air-independent propulsion) technology.
Fact 2: The disaster took place precisely in the waters where what the Pentagon has called "one of the world's largest simulated exercises" was underway. This war exercise, known as "Key Resolve/Foal Eagle" did not end on March 18 as was reported but actually ran from March 18 to April 30.
Fact 3: The Key Resolve/Foal Eagle exercise on the West Sea near the Northern Limit Line (NLL) was aimed at keeping a more watchful eye on North Korea as well as training for the destruction of weapons of mass destruction in the North. It involved scores of shiny, ultra-modern US and South Korean warships equipped with the latest technology.[...]
Fact 4: Deputy Secretary of State James Steinberg said on March 30 that he doubted there was North Korean involvement in the sinking: "Obviously the full investigation needs to go forward. But to my knowledge, there's no reason to believe or to be concerned that that may have been the cause."
General Walter Sharp, US Forces Korea (USFK) commander, also saw no link between North Korea and the sinking. In an April 6 press conference, he said: "We, as Combined Forces Command and the ROK [Republic of Korea] Joint Chief of Staff, watch North Korea very closely every single day of the year and we continue to do that right now. And again, as this has been said, we see no unusual activity at this time."