Say that in response to North Korea's detonation on Monday of a Nagasaki-like nuclear blast, the world declares: "Enough is enough." China, the North's only ally, cuts off fuel and seals its border. South Korea halts humanitarian aid. NATO warships, backed by Russia's Pacific fleet, enforce a total blockade. How might North Korea react? Perhaps by invading the South; perhaps by exploding a nuke over Seoul. Or perhaps the regime would begin to gasp its last, as millions of starving northerners stormed the Chinese and South Korean borders. The sudden, forced reunification of the peninsula would saddle the modern, affluent South with incredible logistical problems - foremost among them how to feed a backward, impoverished population and integrate it into their hyper-modern society. In other words: At this late stage, decision-makers are likely to find meaningful action totally unpalatable. IT HAS no doubt been instructive for Iran's supreme leader Ayatollah Ali Khamenei to observe the civilized world's reaction, over the years, to North Korean provocations. After revving up their nuclear program around the time of the 2003 invasion of Iraq, the North Koreans conducted an underground nuclear test on October 9, 2006. Following an outcry, they pretended to abandon their program and duly obtained a range of economic and diplomatic payoffs. In October 2008, the Bush administration removed North Korea from the State Department's list of countries which sponsor terrorism. Western intelligence analysts think the 2006 test, which registered with the magnitude of a 4.2 earthquake, may have been a dud. As David E. Sanger of The New York Times writes in The Inheritance, "The yield was below a kiloton, far less than a tenth of the power of the bomb dropped on Hiroshima." Not so yesterday's blast, estimated at 4.5 to 5.3 magnitude. North Korea is a hereditary dictatorship led by Kim Jong-Il, who followed his father, Kim Il-Sung. He will likely be replaced by his son Kim Jong-Un. The masses suffer unspeakable deprivation - during the 1990s hundreds of thousands died of famine and disease - while resources are directed to the military-industrial complex and propaganda. A cloying personality cult has transformed the leader into a deity, while xenophobia and racial supremacy lead people to fear change. North Korea purchased centrifuges and, possibly, uranium from Pakistan. It raises money by selling what it knows about building missiles and atom bombs. For 10 years - under the nose of American intelligence - it built Syria's al-Kimbar nuclear facility in the Euphrates Valley, a mirror image, writes the Times' Sanger, of the North's own Yongbyon reactor. As the Bush administration was obsessing over Saddam Hussein's non-existent nuclear weapons, North Korea was preparing to transform next-door Syria into a nuclear power. Mercifully, on September 6, 2007, the nearly completed reactor was smashed by the Israel Air Force before it could be fueled. Pyongyang and Teheran share their nuclear know-how. Iranian scientists were reportedly present at the October 2006 detonation and, we assume, observed Monday's blast too. By learning from the North Koreans, the mullahs may be laying the groundwork - literally - for an underground nuclear explosion of their own. KHAMENEI knows that while the West spins its wheels, Iran's quest for nuclear arms proceeds apace. "Fate changes no man, unless he changes fate," he's fond of telling acolytes. And yet, he can't afford to be sanguine. True, Iran's nuclear sites are scattered throughout his vast territory, providing redundancy and concealment from attack. True, too, complex problems such as fuel-making, bomb-building and weapons delivery are being successfully addressed. Still, the ayatollah worries. It's not inconceivable - even at this late stage - that he could be forced to freeze Iran's program. What if the new American president runs out of patience, sooner rather than later? What if Barack Obama convinces Europe, Russia and China that the world can't afford another North Korea on top of an unraveling nuclear Pakistan? Certainly not one whose imperial ambitions are fueled by high-octane religious extremism. Realistically, Khamenei reassures himself that the prospects of crippling sanctions don't figure even remotely on the international agenda. Still, he will sleep a lot more soundly once stopping Iran becomes as unthinkable as trying to roll back nuclear-armed North Korea.