SEOUL, South Korea — The North Korean artillery barrage on Tuesday on an obscure South Korean island in the Yellow Sea was much more serious than previous naval clashes in the same disputed waters for one basic reason.

"It was the first time they attacked us on land since the Korean War," said Lee Jong Min, professor at Yonsei University and ambassador on security affairs at foreign ministry.

Nobody thinks the attack could have been at the orders of regional commander acting on his own. "Kim Jong Il has to have ordered it," said Robert Collins, retired intelligence analyst for U.S. command here.

The timing was significant — on the first day of South Korean exercises that North Korea has pledged to crush with "relentless retaliation."

North Korea claims the attack was provoked by South Korea for firing first and intruding in its waters. The issue is the Northern Limit Line set by the United Nations Command three years after the Korean War, marking the line in the Yellow Sea below which North Korean boats are banned.

North Korea has been challenging the line for many years, most dramatically in June 1999, when South and North Korean vessels clashed and a North Korean vessel was sunk. And again in June 2002, when a North Korean vessel fired on a South Korean boat, killing six sailors.

The confrontation worsened in November of last year when a South Korean navy corvette sent a North Korean vessel back to port "in flames" with loss of life — though no one knows for sure how many casualties were inflicted. Then, on March 26, a North Korean midget submarine fired a torpedo that sank the Cheonan, a South Korean corvette killing 46 sailors in the worst episode so far in these disputed waters.

Today's episode is more serious than previous incidents in part because North Korea seems determined to pressure the United States and South Korea into going into talks on North Korea's terms.

There seems to be little question that it was deliberately timed to follow up on the disquieting news that North Korea is well on the way to completing a uranium enrichment plant at the nuclear complex at Yongbyon where it's already produced materiel for probably a dozen warheads, according to intelligence estimates.

It's as though North Korea had delivered a one-two punch — first showing off its status as a nuclear power and then putting on a display of its skills in conventional warfare.

One North Korean aim, according to analyst Choi Jin-wook at the Korea Institute of National Unification, is to force the United States and South Korea into negotiations on North Korean terms.

North Korea has no notion of giving up its nuclear program, the topic of six-party talks last held in Beijing in December 2008, but would like to return to the table in order to accomplish other aims, notably withdrawal of the 28,500 U.S. troops, including the powerful U.S. Seventh Air Force, from the Korean peninsula.

North Korea confirmed the planning behind this latest episode with broadcasts cast in rhetoric that people in the South have grown accustomed to ignoring. The fear is that North Korea will step up the attacks while South Korean marines and naval forces go on with exercises that are supposed to last a week.

South Korea's President Lee Myung-bak is mingling tough talk with appeals for calm. He's threatening massive retaliation if North Korea attacks again, but he's also cautioning against extending the conflict.

Lee's top priority here is to maintain calm and head off a conflict that could throw South Korea's booming economy off course. South Korean business people are talking about the impact of the episode on the local stock market while wondering how a widened war will affect business.

South Koreans earlier in the afternoon were crowding around television sets, registering shock and apprehension. But after the firing ceased, the prevailing sense was that now we can get back to work and it probably won't happen again.

Some people, though, were complaining that South Koreans should not show their usual complacency over an incident that carries implications for all of them. Now that North Korea has attacked civilians in a rather remote island of fishermen and farmers in the Yellow Sea, they might take it from there and fire on populated regions.

Seoul, as is often noted, is within range of North Korea's artillery just above the demilitarized zone that has divided the Korean peninsula since the end of the Korean War. The conventional view is that North Korea could wreak complete havoc in Seoul by firing a few artillery pieces, prompting flight from the city, hoarding and thoroughly upsetting the South Korean economy.

“The government stance is this will not lead to war,” said Park Weon-sun, a shopkeeper in central Seoul as people crowded around a TV screen in his shop. “Koreans tend to be more complacent than they should be. I don’t think it has yet really shaken them out of their complacency.”

More such episodes, though, might well have that effect. “Nobody I know has an escape plan,” said Park. “At first every one was shocked but now they are going about business as usual.”

Sooner or later, he said, “People will realize, ‘It can happen to you.’”