Pyongyang's attack has long-term potential to sabotage peace in Asia.
TUESDAY'S deadly barrage by North Korean artillery is more than just another of Pyongyang's grotesque tantrums. It is a critical moment for the US and China, and a test of whether Asia's strategic future will be clouded by war.
How Beijing responds will mark how much - or how little - the US and others can work with it to keep Asia at peace. The biggest danger for Australia and the region from events this week may not be an immediate war in Korea, which seems unlikely, but rather the long term souring of US-China security relations.
Like so much about North Korea, the precise reason for the attack may always be a mystery. The North's batteries unleashed scores of shells against the island of Yeonpyeong, in the midst of an annual defence exercise by South Korean forces. At least four people were killed, many injured, homes destroyed, and a civilian population terrorised.
The assault probably had multiple purposes. The North wants US and global attention. It wants fresh doses of foreign aid for its decrepit economy and a new round of six-nation talks on nuclear issues.
Those talks are meant to be about bargaining an end to North Korea's nuclear weapons program but have instead turned into a stage for Pyongyang to secure new concessions and sow division among its neighbours.
Washington has spurned a return to this discredited diplomatic theatre until the North demonstrates it seriously wants to negotiate. Revelations last weekend about a new uranium enrichment project are just the opposite.
The attack was probably also about demonstrating that the next North Korean leader, Kim Jong-un, knows how to provoke, fight and be as unpredictable as his father, Kim Jong-il. This might secure military support for his impending rule. Indeed, rumours are circulating in Seoul that the elder Kim is already dead.
Another motive may have been to discredit the South Korean government of Lee Myung-bak, which has stood up to the North by joining a maritime coalition to interdict illegal nuclear and missile cargoes.
But whatever the reason it opened fire, North Korea has wrought collateral damage on its only friend, China.
China is the only power with real capacity to inflict peacetime harm on the regime in Pyongyang, as it proved when it cut off energy supplies briefly after the nuclear test in 2006. It is loath to pull the plug again because it fears that instability in its neighbour could hasten regime collapse, refugees flooding into China, and a unified Korea with US forces on the Chinese border.
But doing nothing now will risk turning China's bad image in Asia into a shocking one. Its maritime assertiveness against Japan and in the South China Sea has prompted most of the region to tighten security ties with the United States.
Privately, Chinese security experts admit that Beijing desperately needs a cooling-off period in its relations with Asia and with America after all this year's strife.
The Korean bombardment will make that so much harder. Beijing still absurdly denies that the North sank the South Korean warship Cheonan in March, even though a credible international investigation proved otherwise. But Beijing can hardly dream away the television images of burning South Korean houses. If it takes a business-as-usual approach, its relations with South Korea will be wrecked, and its chances of a working security relationship with the US will be lost, perhaps for years.
In Washington, meanwhile, the tensions will measure the Obama administration's mettle as an ally and protector, not only of embattled South Korea but of Japan, Taiwan, and even Australia.
While the guns boomed across the Yellow Sea, I was in Seoul, convening an expert conference on how the US might best shape its forces to reassure Asian allies without needlessly provoking other powers. It was a sobering exchange. There is a sombre mood in the South Korean security establishment - large-scale conventional war is not a palatable option for a nation with so much to lose.
To be sure, Barack Obama has been quick to reassure his Korean ally. But even before Tuesday's violence, Seoul's defence minister was publicly urging the US to consider redeploying tactical nuclear weapons on South Korean soil - a capability removed at the end of the Cold War.
Such a step would cripple Obama's vision of reducing the role of nuclear weapons. But if it were not to agree, the US will have to talk frankly and secretly with South Korea on precisely how it intends to use its forces in contingencies against the North, a grim conversation it has long been wary of opening.
Otherwise, South Korea - and Japan - may have a quiet rethink about their own defences, even, one day, of the nuclear kind.
To rule this out, there will need to be some smart and robust diplomacy in the present. China will need to radically rethink its Korea policy. And Obama may find that Korea, not Afghanistan, is where his claims of international leadership face their critical trial by fire.
Rory Medcalf is program director for international security at the Lowy Institute. He was in Seoul this week.