It was almost certainly North Korea whose torpedo sank a South Korean warship in the disputed waters of the Yellow Sea. Fourty-six sailors at Cheonan died. Some technical evidence of the statement was found.
Moreover, North Korea has the clear motivation to show that they're still powerful even though everyone ignores them and their compassionate South Korean neighbor wanted to treat them as a small baby which the socialists don't like.
The two countries are still technically in the state of war: the Korean War (1950-1953) wasn't ever ended by any peace treaty. If South Korea decides to retaliate, even by sanctions - and it has declared it wants to retaliate - North Korea (which denies the torpedo attack) wants to enter a full-fledged war.
I think it would be a pretty good idea. This dead socialist spot on the Asian map, led by the Eternal President who has been dead for 16 years, may be violently incorporated into the international community and into the world's market.
The GDP figures clearly show the impact of the disastrous rule of socialism. While the South Korean GDP per capita is $28,000 (PPP), the North Korean figure is only $2,000, fourteen times smaller. South Korea also boasts a greater population, 50 vs 25 million.
Even though the North Korean military power is vastly bigger than the GDP numbers would suggest, it shouldn't be too difficult to defeat North Korea, especially if Japan decided to help, and some persisting worries coming from Asia - including a nuclear worry - could be eliminated. The opportunity won't be as high as now, a few weeks after the sinking of the warship.
And that's the memo.