North Korea warned it would close the last road link across the increasingly tense peninsula if the South goes ahead with a threat to broadcast anti-Pyongyang propaganda into its hermit neighbour.

Tensions are mounting after the South blamed the North for torpedoing one of its warships, killing 46 sailors.

Following is a look at what may have motivated the North to raise the stakes by sinking the South Korean corvette Cheonan and how it may react to the hard line from the conservative South Korean government of President Lee Myung-bak:

REVENGE

One popularly ascribed motive for the March 26 sinking of the Cheonan was payback for a humiliating defeat in a naval clash in November near their disputed maritime border. The South’s navy was operating under new rules of engagement imposed after Lee took office, to strike fast and strike to win decisively.

The humiliation may have been all the greater because the North, and its self proclaimed “invincible” army, got pounded when it may not even have been looking for a fight in the first place. “It’s a case of getting beaten up when they weren’t even being very cocky,” an expert on the North’s propaganda said.

By most accounts, Kim Jong-il would have to have agreed to the torpedo attack. What may have come as a surprise was that the South was able to come up with evidence — some remains of the torpedo — to prove the North’s involvement.

LEADER UNDER PRESSURE

Some experts say that the attack seems to have been disproportionate to the North’s losses in the November skirmish, especially as most North Koreans would have had no idea the clash had even taken place, and certainly not that it lost.

One explanation is that the reclusive Kim, known at home as the “Dear Leader”, is struggling to secure the succession of his youngest son to head the family dynasty that has run the North since its founding after World War Two.

As a result, he needs to display his strength, especially to the military elite that he has nurtured and put at the top of society’s hierarchy.

Kim himself looks in poor health after an apparent stroke nearly two years ago. His government also reportedly faced rare public unrest after a disastrous change in the value of the currency late last year forced the closure of private markets, which help make up for the state’s inability to supply its own people with enough food.

Dictatorships undergoing internal political turmoil tend to manifest disproportionately belligerent behaviour to the outside world, said Victor Cha, a U.S. expert who had been involved in negotiations with the North.

EXTORTION

North Korea has often staged provocative incidents as a way to get back to the negotiating table with the South and regional powers to extract economic and political concessions.

If this was the motive, then it backfired. Whatever inclination there may have been to bring the six regional powers back together to formulate a massive package of aid to the North in return for Pyongyang’s promise to dismantle its nuclear arms programme all but disappeared with the sinking of the Cheonan.

Kim Jong-il’s interest may have been more in separate talks with the United States to discuss a permanent peace treaty to replace the armistice that ended fighting in the 1950-53 Korean War, than with the group hosted by China and also involved South Korea, Japan and Russia.

Some analysts and defectors from the North say the leaders in Pyongyang have a genuine fear of an invasion by the United States launched from the soil of its ally, South Korea. There is also huge mileage for domestic propaganda purposes in telling its public that it was negotiating with the United States on equal footing. Staging a deadly attack in the waters near a naval border it had disputed gives the North’s military an excuse to demand talks on ending a truce.

PEACE TREATY

This is a variation on the above scenario, with the difference that the North is looking for a security framework instead of aid. The Cheonan sinking is the latest in a series of incidents along the disputed maritime border in the Yellow Sea, including an exchange of artillery fire in January.

Kim Jong-il may be hoping to goad the United States into taking more seriously his demands to agree finally a peace treaty to end formally the 1950-53 Korean War. Washington has been reluctant to be lured into those talks, arguing the North must first give up its efforts to build nuclear weapons.

Much of the justification for his iron rule, and extreme poverty that faces most of his population, is that it is the only way to keep a belligerent United States at bay. A peace treaty would not only allow him to stop raiding his depleted treasury to pay for one of the world’s largest standing armies, some analysts say it would also open the way to international financial aid for his broken economy.

The peninsula remains in a technical state of war because the Korean War ended in a truce, not a peace treaty. If Kim keeps making the Yellow Sea border — drawn unilaterally by the U.S.-led United Nations Command at the end of the war — a combat zone, maybe that would eventually lead to peace treaty talks. After all, previous instances of North Korean misbehaviour resulted in negotiations that led to benefits.

ARMS SALES DEMO

North Korea depended heavily on exports of missile and artillery parts for a large part of its income before U.N. sanctions last year for testing a nuclear device sharply cut off its trade. It may have wanted to demonstrate its capabilities in submarine and torpedo warfare.

(Editing by Jonathan Thatcher and Bill Tarrant)