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Against opaque attacks - which could undermine international investors’ confidence in the South’s economy and would test the leadership of President Lee Myung-bak — retaliation options are limited.

“A secret attack, which disguises the author of the crime, is very hard to identify,” said Mr. Choi. “In that kind of case, it is very difficult to answer what we would do.”

The South has been historically unable to respond effectively against such operations, but if the KPA attacks openly — as in November’s shelling of Yeonpyeong Island, during which four people died — South Korean Defense Minister Kim Kwan-jin has indicated he would upgrade retaliation.

The danger would be whether the North would then retaliate against the response, sparking a cycle of escalation.

“If you are the South, you have to have escalation dominance. It must be clear to North Korea that they are going to pay a higher price,” said Mr. Pinkston. “I am not confident that the South is prepared for that.”

Seoul is disadvantaged in that situation: It has to safeguard its economy, which is subject to global capital markets; it is beholden to its people and to international pressure; and it has to answer to its public for the lives of its own troops. None of these conditions hinder Pyongyang’s freedom of action.

Still, South Korean public opinion has hardened against the North.

“After the Cheonan, we saw division in South Korean public opinion, but after Yeonpyeong, the public felt very different,” said Kim Tae-woo, of the Korea Institute of Defense Analyses. “There was a feeling that patience is not the way to protect the national interest.”

There was public uproar over Seoul’s ineffective response to the Yeonpyeong shelling. Satellite photos indicate that South Korean counterbattery fire against KPA artillery did not hit any of the launch systems involved in the fatal barrage.

The South is refortifying its five front-line islands close to the disputed “northern limit line” in the Yellow Sea, and the defense minister has strongly hinted that future southern retaliation would include airstrikes.

“If they attack again, we will retaliate without fail,” Mr. Kim said. “And that has the possibility to trigger bigger conflict.”

A spiraling escalation would raise the nightmare scenario of the resumption of full-scale hostilities on the Korean Peninsula. North Korea has threatened to use nuclear weapons if war breaks out. Global financial markets would be devastated in a Korea-based conflict because Northeast Asia is the world’s third-largest zone of economic activity, after North America and Western Europe.

What’s more, the United States, signatory to a mutual defense treaty with South Korea, would be dragged in, and U.S. naval and Marine personnel in Okinawa and Tokyo also could be involved.

The KPA’s Achilles’ heel might be its conventional forces’ command and control and its offensive sustainability.

“They know that their division is tasked to take that hill and that town — day one, day two, day three — until they get to Busan [South Korea],” said Mr. Bermudez. “What happens when this gets all screwed up? Their command-and-control network is weak and, as best we can tell, their greatest weakness is their inability to handle the unexpected.”

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