Cheonan was likely sunk by a mine or torpedo. I would be betting very heavily on “torpedo” at this point; no mine from the first Korean War has been discovered since 1986, and a new mine doesn’t make a lot of sense (a single mine would have to be fantastically lucky, and no other mines appear to have been found).
There are a few ways to read this. The first, unproblematic story that we’ll probably get in the Western media is about unprovoked North Korean aggression resulting in the deaths of dozens of South Korean sailors. There’s something to this, but it isn’t quite right; the area in which Cheonan was sunk is within UN-demarcated South Korean waters, but the North Koreans have never respected the UN drawn line, and the UN was, after all, one of the belligerents in the original conflict. Moreover, this area has seen a number of skirmishes between North and South Korean naval forces over the past decade, including several within the last year. The second story, thus, will be of a steadily escalating conflict in which the North Koreans just decided to significantly up the ante. Targeting a South Korean patrol boat with a torpedo is rather different than the exchanges of light gunfire that have taken place before; it indicates the intent to destroy the South Korean ship, likely with large loss of life. There’s also a story-within-a-story here about the North Korean decision making process. It’s simply not the case that the order to escalate MUST have come from Kim Jong Il, although it’s certainly POSSIBLE that it came from the top.
The South Koreans are pursuing this very cautiously, and the Americans have been following the South Korean lead. Indeed, I’m guessing that, official determination or no, it was probably obvious from quick visual inspection whether a mine, torpedo, or internal explosion caused the sinking. This caution, I think, effectively rules out a “Gulf of Tonkin” scenario. Here’s my question; what ought the South Korean reaction be? There’s a wide spectrum of bad behavior that the North could engage in, and isn’t at the edge of that spectrum; the North didn’t, for example, torpedo Dokdo in the midst of humanitarian operations in international waters. At the same time, while I’m skeptical of “message sending,” indicating that North Korea is allowed to sink South Korean ships in internationally recognized South Korean waters seems problematic.