BARACK OBAMA began his presidency with an impassioned plea for a world without nuclear weapons. This week, in his last year in office (and as we went to press), he was to become the first American president to visit Hiroshima, site of one of only two nuclear attacks. Mr Obama has made progress on nuclear-arms reduction and non-proliferation. He signed a strategic-arms-control treaty (New START) with Russia in 2010. A series of nuclear-security summits helped stop fissile material getting into the wrong hands. Most important, he secured a deal in July to curtail and then constrain Iran’s nuclear programme for at least the next 10-15 years.

But in one area, his failure is glaring. On Mr Obama’s watch the nuclear-weapons and missile programme of North Korea has become steadily more alarming. Its nuclear missiles already threaten South Korea and Japan. Sometime during the second term of Mr Obama’s successor, they are likely also to be able to strike New York. Mr Obama put North Korea on the back burner. Whoever becomes America’s next president will not have that luxury.

The other Manhattan project

The taboo against nuclear weapons rests on three pillars: policies to prevent proliferation, norms against the first use of nukes (especially against non-nuclear powers) and deterrence. North Korea has taken a sledgehammer to all of them.

No country in history has spent such a large share of its wealth on nuclear weapons. North Korea is thought to have a stockpile of around 20 devices. Every six weeks or so it adds another. This year the pace of ballistic missile testing has been unprecedented (see article). An underground nuclear detonation in January, claimed by the regime to be an H-bomb (but more likely a souped-up A-bomb), has been followed by tests of the technologies behind nuclear-armed missiles. Although three tests of a 4,000-kilometre (2,500-mile) missile failed in April, North Korean engineers learn from their mistakes. Few would bet against them succeeding in the end.

North Korea is not bound by any global rules. Its hereditary dictator, Kim Jong Un, imposes forced labour on hundreds of thousands of his people in the gulag, including whole families, without trial or hope of release. Mr Kim frequently threatens to drench Seoul, the South’s capital, in “a sea of fire”. Nuclear weapons are central to his regime’s identity and survival.

Deterrence is based on the belief that states act rationally. But Mr Kim is so opaque and so little is known about how decisions come about in the capital, Pyongyang, that deterring North Korea is fraught with difficulty. Were his regime on the point of collapse, who is to say whether Mr Kim would pull down the temple by unleashing a nuclear attack?

The mix of unpredictability, ruthlessness and fragility frustrates policymaking towards Mr Kim. Many outsiders want to force him to behave better. In March, following the recent weapons test, the UN Security Council strengthened sanctions. China is infuriated by Mr Kim’s taunts and provocations (it did not even know about the nuclear test until after it had happened). It agreed to tougher measures, including limiting financial transactions and searching vessels for contraband.

But China does not want to overthrow Mr Kim. It worries that the collapse of a regime on its north-eastern border would create a flood of refugees and eliminate the buffer protecting it from American troops stationed in South Korea. About 90% of North Korea’s trade, worth about $6 billion a year, is with China. It will continue to import North Korean coal and iron ore (and send back fuel oil, food and consumer goods) as long as the money is not spent on military activities—an unenforceable condition.

Protected by China, Mr Kim can pursue his nuclear programme with impunity. The sanctions are unlikely to stop him. If anything, they may spur him to strengthen and upgrade his arsenal before China adopts harsher ones.

Understandably, therefore, Mr Obama has preferred to devote his efforts to Iran. Because the mullahs depend on sales of oil and gas to the outside world, embargoes on Iran’s energy exports and exclusion from the international payments system changed their strategic calculus. But this logic will not work with North Korea.

Can anything stop Mr Kim? Perhaps he will decide to shelve his “nukes first” policy in favour of Chinese-style economic reform and rapprochement with South Korea. It is a nice idea, and Mr Kim has shown some interest in economic development. But nothing suggests he would barter his nuclear weapons to give his people a better life.

Perhaps dissent over Mr Kim’s rule among the North Korean elite will lead to a palace coup. A successor might be ready for an Iran-type deal to boost his standing both at home and abroad. That is a possibility, but Mr Kim has so far shown himself able to crush any challengers to his dominance.

The last hope is that tougher sanctions will contribute to the collapse of the regime—which, in turn, could lead to reunification with the South and denuclearisation of the Korean peninsula. That would be the best outcome, but it is also the one that carries the most danger. Moreover, it is precisely the situation China seeks to avoid.

Fat boy

Without any good options, what should America’s next president do? A priority is to strengthen missile defence. New THAAD anti-missile systems should be sent to South Korea and Japan, while America soothes objections that their radar could be used against China’s nuclear weapons. China should also be cajoled into accepting that sanctions can be harsher, without provoking an implosion. Were that to lead initially only to a freeze on testing, it would be worth having. Because a sudden, unforeseen collapse of Mr Kim’s regime is possible at any time, America needs worked-out plans to seize or destroy North Korea’s nuclear missiles before they can be used. For this China’s co-operation, or at least acquiescence, is vital. So clear and present is the danger that even rivals who clash elsewhere in Asia must urgently find new ways to work together.