How significant is North Korea's shelling of Yeonpyeong island?

Since the 1953 truce between North and South Korea, this is the first time that North Korea artillery has landed on South Korean territory.

This attack may be unprecedented but there have been more than 150 incidents between the two Koreas since 1953. (Technically, they are still at war since no peace treaty has been signed since the Korean War began in 1950.)

Where is Yeonpyeong?

The island is 115 kilometres northwest of Seoul, the South's capital, 11 kilometres off the coast of North Korea and just 1.6 kilometres from the Northern Limit Line (NLL).

The NLL is the sea border between the two Koreas, declared in 1953 by the U.S.-led UN forces.

Yeonpyeong's only regular link to the south is via a 106-kilometre ferry ride.

The island has about 1,600 residents – mostly fishermen and their families –plus another 1,000 troops on its eight square kilometres.

What is Yeonpyeong's status?

The island is internationally recognized as part of South Korea, but the North disputes the claim.

North Korea also does not recognize the NLL.

What happened in previous fighting around Yeonpyeong?

The South Korean navy raises the bullet-riddled South Korean patrol boat that was sunk during a June skirmish with North Korea in the Yellow Sea off Yeonpyeong island, on Aug. 21, 2002. The battle along a disputed sea border left four South Korean sailors and 13 North Koreans dead. ((Kim Jae-hwan/Reuters))

There were two previous battles, in 1999 and 2002.

The 1999 battle followed North Korean attempts to redraw the NLL. That conflict escalated into a major naval battle on June 15 in which 30 North Korean sailors died, none from the South. After the battle, North Korea resumed respecting the NLL.

On June 29, 2002, two North Korean patrol boats crossed the NLL, leading to a battle with two patrol vessels from the south. Thirteen sailors on the North's boats and four from the South died.

The battle ended when Southern reinforcements showed up, prompting the North's boats to flee to their side of the NLL.

Because the waters near the NLL are a rich crab fishing area, these battles are sometimes referred to as the crab wars.

Was this latest conflict a surprise?

For the island's residents, it was a clash waiting to happen. Martin Fackler reported from Yeonpyeong for the New York Times in June 2009 that it was "feared a new battle could erupt at any moment." The islanders had been conducting monthly air raid drills.

Earlier that year, the North began firing artillery into the Yellow Sea near Yeonpyeong. That was for training, which continued until at least August 2010.

People stand near destroyed houses on Yeonpyeong island in South Korea on Wednesday. North and South Korea exchanged artillery fire Tuesday after the North shelled Yeonpyeong near their disputed sea border, killing at least two South Korean marines. ((Yonhap/Associated Press) )The surrounding waters are also a favourite for U.S.-South Korea military drills, each one denounced by the North.

Tensions have been building, especially since March, when the South Korean warship Cheonan sank following an explosion. A South Korean investigation, with international backing, concluded the explosion was a result of a torpedo attack by the North, which North Korea denies. Forty-six sailors died.

In October, the North proposed holding talks to address anti-North Korean leafleting via balloons from the South. The North warned of "merciless physical retaliation" for not accepting the talks. South Korea rebuffed the North's proposal.

On Oct. 29, shots were exchanged across the demilitarized zone separating the two Koreas.

Stanford University scientist Siegfried Hecker speaks about his recent trip to North Korea, Nov. 23, at the Korea Economic Institute of America in Washington. (Alex Brandon/Associated Press)

Shortly after that, a South Korean naval vessel fired warning shots at a fishing boat from the North that had crossed the NLL.

Also, the North Koreans recently showed visiting U.S. scientist Siegfried Hecker a new uranium enrichment facility where they claimed they had 2,000 centrifuges installed and running. Hecker told the New York Times he was "stunned by the sophistication of the new plant."

That revelation led South Korea's Defence Minister Kim Tae-Yong to raise the possibility of the U.S. redeploying tactical nuclear weapons on its territory. (The U.S. withdrew such weapons in 1991.) Just hours before the artillery attack, the president's office in Seoul said the issue was "not a subject of discussion."South Korean women pick crabs from a mesh of a net on a crab fishing boat at Yeonpyeong island in June 2002. Things can get edgy as fishermen from North and South ply the waters off the island. ((Katsumi Kasahara/Associated Press))

What were the immediate events leading up to the shelling of Yeonpyeong?

On Monday, Nov. 22, the South's navy began conducting its annual military drill, called "Safeguarding the Nation." As usual the North protested, saying it was "a means to provoke a war" and protested again on Tuesday morning.

On Tuesday, the South said it was conducting a routine firing exercise near Yeonpyeong.

According to a statement issued by the North: "The South Korean enemy, despite our repeated warnings, committed reckless military provocations of firing artillery shells into our maritime territory near Yeonpyeong island." That happened about 1 p.m. local time, according to the North.

The South says it was firing into the sea to the west, not north.

Soldiers from a South Korean army armoured division take part in an annual river-crossing exercise against a possible attack from North Korea on the Han river in Yeoju, about 100 km southeast of Seoul, Nov. 23. (Jo Yong-Hak/Reuters)

At 2:34 p.m., artillery shells began falling on and around Yeonpyeong. The firing – about 170 artillery shells in all – continued until 4:42 p.m. About 80 rounds landed on the island, many of them hitting an army barracks. Two marines were killed and 15 injured. Two construction workers and three other civilians were also injured.

The North once again threatened "merciless military retaliation," this time if the South intrudes into their territorial waters.

How did South Korea respond to the North's artillery attack?

South Korean military officials told the Yonhap News Agency that, "South Korea immediately returned fire and lobbed more than 80 shells toward North Korean artillery positions on the west coast and sent fighter jets to the island."

South Korean marines salute their fellow soldiers killed in a North Korean bombardment of Yeonpyeong island during a memorial service at a military hospital in Seongnam, South Korea, Nov. 24. (Shin Young-gun/Yonhap/Associated Press)

While emphasizing they wanted to avoid further escalation, the South talked about "a much harder military response" if the North attacks again. Chung Min Lee of the foreign ministry said that could include "taking out their artillery sites with our aircraft."

The South indefinitely postponed a Red Cross meeting about family reunifications.

What explanations are on offer for North Korea's actions?

Interpreting any major action by North Korea is a challenge. Here's a sampling of expert opinion:

• Sue Mi Terry, Council on Foreign Relations, New York: "What North Koreans hate is being ignored. So what they really want right now is to direct attention to themselves by saying 'look at us, remember we're dangerous.' This artillery attack is the latest in a series of provocations." She added, "North Koreans love to escalate, create tensions, then bring us back to the negotiating table and then reap concessions." (CBC, As it Happens) Smoke raises from South Korea's Yeonpyeong island, Nov. 23. North Korea shot dozens of rounds of artillery onto the populated South Korean island near their disputed western border Tuesday, military officials said, setting buildings on fire and prompting South Korea to return fire and scramble fighter jets. (Yonhap/Associated Press)
• Chung Min Lee, ambassador for International Security Affairs and Global Issues, South Korea: "There have been reports of sporadic opposition to [North Korean leader] Kim Jong-un's assumption of power and Pyongyang may have been motivated by instilling fear in its population during a period of critical succession." (Reuters)
• Zhang Liangui, a North Korea expert at the Central Party School, Beijing: "Kim Jong-un was deliberately destabilizing the environment in order to mobilize the military and consolidate his power." (Sydney Morning Herald)
• Robert Kelly, Pusan National University in South Korea: "My primary guess is that this is a response to the recent international prestige taken by South Korea at the G20." (BBC)
• Yang Moo-jin, University of North Korean Studies, Seoul: "North Korea didn't get the desired effect when it made its nuclear facility public. South Korea didn't back down from its hardline policy." (Los Angeles Times)