Kim Jong Il has gone for broke. North Korea’s claim to have conducted its first nuclear test has yet to be fully verified. It is still unclear whether the country is years, or perhaps only months, away from developing a missile-delivered nuclear warhead. But by declaring itself to be set on being a nuclear-armed power, North Korea has gone past the point of brinkmanship, leaving no shred of doubt about its intentions — and no room for ambiguity in the international response.

As the new Japanese Prime Minister Shinzo Abe grimly observed yesterday, Asia’s most untrustworthy regime has launched the region into “a new, dangerous nuclear age”. North Korea could not have been more sternly warned that if it went ahead with the test, it would find itself in a break-hold from which it could not escape. The regime has escaped total isolation until now, mainly because of the reluctance of China, North Korea’s economic lifeline and ultimate protector, to admit to, or use, its considerable economic and political leverage. But even China’s patience has been wearing thin. It pointedly declared at the UN last week that “no one is going to protect . . . bad behaviour”. To underline that message, Beijing told Pyongyang that it might drop the “automatic intervention” clause in the friendship treaty that commits China to come to the regime’s defence.

China has cut back food aid to North Korea in recent months and quietly co-operated with US efforts to destroy the regime’s lucrative counterfeiting industry, which includes renminbi as well as dollar notes. Now, faced by the embarrassing and undeniable failure of its strategy, China must take the lead in ensuring that defiance does not pay — which means demonstrating that North Korea has made itself more, not less, vulnerable by going nuclear. The shock of what is perceived as “betrayal” has forced South Korea to abandon its so-called sunshine policy; where others lead, Seoul will perhaps follow, though it will still be tempted to play a role as a “go-between”, even though it is really between a rock and a nuclear place.

The question is what will work. The regime, interested only in its own survival, is prepared to let millions starve. Yet North Korea, as Beijing understands better than most, is vulnerable to sustained pressure. The $2.5 billion a year in Chinese and South Korean trade and aid is critical to the elite’s comfort. Mr Kim must have known that North Korea would be severely squeezed, at least in the short term, by external financial, trade, aid and investment sanctions, not least a cut-off of Chinese oil. That suggests that this decision, like the conducting of missile tests earlier this year, may have been dictated by even greater pressures building up internally. The regime has always relied on a siege mentality to rally loyalty and keep its all-powerful military in line; this signal act of defiance may be aimed at compelling the military to close ranks.

The generals must be given fresh reasons for discontent. There will be members of the elite whose stomach is turned by Korean suffering and the crass Kim personality cult. China knows who these leaders are. Beijing would balk at “regime change”; but perhaps not at edging out the Dear Leader in favour of pragmatists ready to risk accommodation and cautious reform. Mr Kim’s departure is the neatest way out of crisis. He may have hastened the forced exit he is desperate to prevent.