Leading article: Aggression that cannot simply be ignored

Yesterday's exchange of artillery fire across the border between the two Koreas marked one of the worst clashes since the war half a century ago. And yet almost everyone is desperate that it shouldn't be so and that the outburst should be a one-off, reflecting internal North Korean politics rather than expansionary ambitions. The feeling is understandable. The last thing anyone wants at this time – not China, nor Japan nor the West – is a full-blown conflict between a nuclear-armed failing regime and its US-backed, fast-growing neighbour to the south.

There are all sorts of reasons why the clash can be explained away in terms of short-term politics. An ailing North Korean president, Kim Jong-il, is anxious to prepare the way for the succession of his youngest son, Kim Jong-un, and may be keen to get the military on his side. Alternatively, the military may be flexing its muscles in preparation for that handover. At the same time, North Korea may also feel pressured by the military exercises conducted on its border this month and the recent meeting of G20 world leaders that was held in Seoul.

Yet the desire for a quieter life cannot be allowed to evade the fact that North Korea has deliberately violated the laws of international relations by a succession of incidents in which it has torpedoed a South Korean naval vessel, with the loss of 46 lives, and now opened fire not just on the military installations but on civilians in Yeonpyeong. It has also now revealed, with deliberate timing, a uranium enrichment programme which it had kept secret from the world.

The West should have learnt enough from its experience in Iraq to know the limits of what it can, or should, do about the government in Pyongyang. But it must make clear, to China as much as to North Korea, that it will stand absolutely by its defence commitments to South Korea, that these sorts of conflicts should be resolved through negotiation not violence and that the way forward has to be in closer relations between the two divided parts of the peninsula. It is no more in Beijing's interests than in Washington's that armed conflict between North and South should escalate.