SEOUL North Korea on Tuesday threatened military action if the South continued to violate its waters off the west coast, arguing the peninsula remained in a state of war because no peace treaty was signed after the 1950-53 Korean War.
The threat adds to escalating tension on the peninsula after the South accused its impoverished neighbor of sinking one of its warships in March, killing 46 sailors.
Following are some questions about the peninsula and why Kim Jong-il may be trying to pick a fight now.
AFTER 57 YEARS OF TRUCE, WHY DOES "STATE OF WAR" STILL
The Korean War ended in 1953, but the peace never arrived.
Troops laid down their arms on July 27, 1953, three years after North Korean forces crossed the border on a Sunday morning and began fighting in a conflict that would kill 140,000 South Korean, 36,000 U.S. soldiers and 1 million civilians.
The Armistice Agreement was signed by North Korea's Kim Il-sung, state founder and the father of the current leader Kim Jong-il, China -- which had joined the war to back Pyongyang -- and a U.S. general representing the United Nations Command.
The truce still stands today. Over the years, the North and the South have signed a series of agreements on non-aggression and non-interference. In 2000, a landmark pledge called for an end to decades of hostility and for peace and cooperation, including commercial projects that would become the source of hundreds of millions of dollars a year for the North.
When a North Korean submarine torpedoed a South Korean navy ship in March, President Lee Myung-bak's government decided that the sprit of those agreements was damaged beyond repair.
He said on Monday it had broken the Armistice Agreement.
The South has put up with years of provocations by the North, including a bombing in Myanmar in 1983 that killed several of its cabinet ministers and the bombing of a Korean Air flight that killed 115 on board in 1987. There have also been deadly naval clashes on their disputed sea border. The South had kept up trade, tourism and humanitarian projects after those incidents but all that will change, it says now.
WHAT IS THE NORTHERN LIMIT LINE?
The U.S. commander at the end of the Korean War drew up a sea border weeks after the Armistice was signed to prevent naval clashes off the peninsula's west and east coasts. The truce made no direct mention of sea borders.
The NLL off the west coast, as the maritime border is called, is drawn around islands that lie well north of what would be the extension of the land border that had been part of the South's territory before the war.
The North made no issue of the sea NLL until 1973, when it began violating the limit and disputing its validity. An agreement signed in 1991 reaffirming non-aggression and recognizing each other's political sovereignty appeared to settle the maritime dispute.
In the 1990s, the North again began disputing the NLL, claiming the real border should lie far to the south. In 1999, a North Korean patrol boat violated the NLL by as much as 10 km (6 miles) but went home defeated when a gun fight killed several North Koreans.
A gun fight near the same area three years later killed six South Korean seamen. In November 2009, South Korean navy ships responding quickly to an intruding North Korean vessel pounded it with thousands of rounds of gunfire, disabling it with several crew believed killed.
HOW FAR HAVE NORTH AND SOUTH KOREA DRIFTED APART?
In the aftermath of the 1950-53 Korean War, the North's economy developed more quickly, but in the past few decades South Korea has emerged as an economic powerhouse while the North has become a backwater with an annual GDP of $20 billion -- three percent the size of the South's economy.
The South has about 20 nuclear power plants while the North relies on an electrical grid mostly built during Japan's 1910-45 colonial occupation of Korea and cannot produce enough power to keep the lights on at night.
South Korean estimates have said it would cost $1 trillion or more to absorb the North in the event of reunification.
WHY IS KIM JONG-IL PICKING A FIGHT NOW?
North Korea is angry at the government of President Lee, who stopped a decade of aid and this week shut down trade between the two Koreas that is worth close to $300 million in hard cash.
Analysts say the North's leaders often resort to raising regional tensions to divert attention from troubles at home. Kim, whose own health is in question, is trying to promote his youngest son as heir.
There is concern in the South that Kim may be inclined to more lethal provocations because the routine saber-rattling of recent years no longer seems to work to force concessions out of the South and regional powers.
(Editing by Jonathan Thatcher)