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Why did this happen?

Tension has been running particularly high in the Korean peninsula after the March 26 sinking of the South Korean warship Cheonan. Tuesday's incident, however, is one of the most serious that has occurred in recent years.

The hostilities come as North Korea is undergoing transition -- the ailing and reclusive leader Kim Jong Il is believed to be in the process of transferring power to his son Kim Jong Un. Some analysts believe upcoming internal changes have prompted North Korea to flex its military muscle in recent days.

Tuesday's violence was also preceded by the revelation of a North Korean uranium enrichment program.

Has this happened before?

Yes, Yeonpyeong Island has come under attack before. Last January, South Korea reported that the North had fired shells that fell in waters north of the Northern Limit Line, the de facto inter-Korean maritime border.

North Korea wants that border redrawn farther south.

Over the past six decades, small-scale skirmishes have flared repeatedly along both land and sea borders as each state aims to reunify the peninsula according to its own terms and system of government. Deadly naval clashes occurred along the demarcation line in 1999, 2002 and 2009.

Will the two nations go to war?

South Korea put its military on high alert following Tuesday's exchange of fire. But whether that will translate into further military action is impossible to predict.

Events in the past few months suggested a slight thawing of icy relations.

North and South Korea had begun discussions on the possible resumption of reunions of family members separated by the Korean war and North Korea has requested military talks. In early September, the South offered food aid to the impoverished North for the first time in three years.

Given the closed nature of North Korean politics, it's hard to tell what changes the new leadership of Kim Jong Un will entail or whether re-engagement is on the table. Another wild card is the influence of China; some South Koreans fear a Chinese takeover in the event of a North Korean collapse.

Some analysts viewed Tuesday's exchange as North Korea flexing its military muscle in the light of its leadership transition. Others said it was related to the nuclear issue.

How will nuclear talks be affected?

Washington accuses Pyongyang of running a secret uranium-based nuclear program. The United States, along with the two Koreas, Russia, Japan and China, have been involved in what is called the Six Party Talks.

But those talks have been slow and arduous and in limbo since 2008. And after the revelation of the North Korean uranium enrichment facility a few days ago, the resumption of talks seemed in jeopardy.

Stephen Bosworth, the U.S. special envoy on North Korean denuclearization, said Tuesday's hostilities will prove a further obstacle.

Choi Jin-wook, senior researcher at the Korea Institute of National Unification, said the North is "frustrated with Washington's response to their uranium program and they think that Washington has almost given up on negotiations with North Korea."

"I think they realize they can't expect anything from Washington or Seoul for several months, so I think they made the provocation," Choi said.

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The first responsibility of the United States must remain the preservation of deterrence from outright return to war. The attacks leave the alliance with few good options for reprisal without risking a far less desirable renewal of conflict.

The United States must work with South Korea to bolster deterrence, check North Korean aggression and be ready to turn it back, but only if absolutely necessary. We will also have to operate without fully understanding the power at play in North Korea.

This year has already been a deadly one as an ailing Kim Jong Il accelerated the likely dynastic transition in power to an ill-prepared son who will be captive to aging generals in the Korean People's Army. In April, 46 South Korean crew members aboard the naval vessel Cheonan died when the vessel was ripped in two by a North Korean torpedo.

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