The FT has a good summary of the possible reasons behind the North Koreans deliberate sinking of the South Korean warship on March 26.

● Revenge
North Korea wanted revenge for a sea battle in November, when one of its ships was badly damaged. The vessel had exchanged fire with South Korean gunboats after straying across a disputed maritime border into what Seoul insists are its waters. South Korean military intelligence says revenge is Pyongyang’s primary motive.

● To smooth the succession
Kim Jong-il, North Korea’s dictator, is almost certainly transferring power to his third son, Kim Jong-un. Some defectors have said he is trying to associate Jong-un’s name with major successes in domestic propaganda. One civic group with contacts in North Korea says celebrations at a naval base directly honoured Jong-un for the sinking.

● An internal power struggle
Some analysts believe the attack could have been the work of a single rogue commander, possibly vying for patronage as the succession gathers pace. North Korea this month made the highly unusual announcement that it was removing Kim Il-chol, a senior admiral on the National Defence Commission, prompting speculation the navy could have exceeded its authority. But most mainstream analysts think the attack would have been impossible without the green light from Kim Jong-il.

● A reversion to hardline ideology
Some scholars say Kim Jong-il had, until last year, been increasingly open to advice from a more liberal faction, advocating market and currency reform. When this backfired, he had no choice but to listen more to Cold War-era ideologues who favoured tactics such as assassinations and submarine attacks. Seoul’s spy service has also recently detained assassins sent into South Korea to assassinate the most senior communist official to flee to the south.

● Breakdown of command in North Korea
Perhaps the most worrying of the possibilities is that Kim Jong-il is no longer in full command, possibly because of a stroke the North Korean leader suffered in 2008. This could mean the sinking was either the result of jostling commanders or poor judgment from Mr Kim himself. Andrei Lankov, a North Korea expert at Kookmin University in Seoul, believes the country has become a “rudderless ship” and that logical decision-making has fallen to pieces, as seen when Pyongyang revalued its currency to disastrous effect late last year.

● To distract from economic woes at home
South Korea’s military intelligence argues the sinking of one of its warships by Pyongyang could distract from hunger and economic failure in the north. However, North Korea openly denies the attack and there is little evidence of it being mentioned in propaganda beyond that intended for small numbers of senior military and Communist party officials.

● Bitterness about G20 meeting in Seoul
Seoul has been turning its presidency of the G20 group of leading economies this year into domestic propaganda, parading how well it has developed economically since the Korean War of 1950-1953. Pyongyang has historically been resentful of South Korea hosting major international events such as the Olympics and the World Cup and has tried to wreck any positive press.