The recent attack by North Korea on the South on Yeonpyeong Island was deliberate, calculated and designed to send a clear message that can be summed up best in four words "BACK THE HELL OFF"!
South Korea has been conducting military exercises recently in the "disputed zone", near the island and engaging in such provocative acts as firing artillery shells in the direction of the North Koreans then claiming that their shells fell inside international waters. This has happened on numerous occasions.
In response North Korea has repeatedly warned the South against conducting military exercises too close to it's territory, especially near the disputed areas. The South for whatever reason ignored those warnings time and time again.
Even after the sinking of a South Korean warship last March it should have been clear that the North Koreans are not playing games here!
Why Yeonpyeong Island?
Yeonpyeong is a minor island located only 7.5 miles off the coast of North Korea. It's not really strategically imporant but it is close in proximity, so the South Koreans decided to position artillery and gun batteries there.
Both countries claim the Island is within their territory by the way and there is no relevant international law to settle this dispute. In such a situation different nations claim the right to different distances that can be attributed to national territory at sea. So you can see how disputes can happen here.
And this isn't the first time something like this has happened. Another icident took place here in 1999 - known as the "first Battle of Yeonpyeong Island", when North Korea wanted to change the demarcation lines in the seas. Soldiers on both sides were killed in than incident as well.
It's seem clear to me that both sides are playing a dangerous cat and mouse game in the disputed areas with each other that could easily trigger another world war.