WHENEVER North Korea causes trouble, politicians in America and elsewhere point fingers at its only ally, China. So on April 7th, amid a daily torrent from Pyongyang of threats of war and nuclear annihilation, John McCain, a Republican senator, told an American television programme that Chinese behaviour was "very disappointing." Senator Charles Schumer, a Democrat, agreed, saying it was about time the Chinese "stepped up to the plate and put a little pressure on this North Korean regime." Yet there is another school of thought: that China, exasperated with North Korea's lunatic provocations, is a calming influence and is tacitly co-operating with America on easing tensions. Certainly, China looks as if it is applying pressure. It seems to hold the North's jejune despot, Kim Jong Un, in even lower esteem than it did his predecessor and father, Kim Jong Il. It signed up to UN Security Council resolutions tightening sanctions on North Korea both in January, to punish it for a rocket launch in December, and in February, after its third test of a nuclear device. And, as North Korea in March declared the armistice that ended the Korean war in 1953 nullified, China - also a signatory - reaffirmed its own commitment to the armistice. Some close to the American administration do detect a change in China's attitude. Kurt Campbell, who until February was the assistant secretary of state for East Asia, said this month that the Chinese government had realised it had to be much clearer with North Korea that what it is doing is "undermining China's security," and that it was asking: "If this is a buffer state, what's it good for?" Moreover, China, which normally bristles at manifestations of America's "pivot" of its military might to Asia, has been remarkably low-key in its reaction as America has moved warships, missile defences and planes, including nuclear-capable bombers, to the region in response to mounting tensions. Yet installing missile defences on Guam, for example, also has an impact on the effectiveness of China's own nuclear deterrent. Zhu Feng, a scholar at Peking University, has argued that North Korea offers "a new platform for China and the United States to get closer." The notion of North Korea as a cockpit of Sino-American co-operation seems rather far-fetched. But a succession of senior Americans is about to visit Beijing. John Kerry, the secretary of state, arrives on April 12th. A week later he will be followed by General Martin Dempsey, chairman of the joint chiefs of staff, on a mission to improve communication between American and Chinese armed forces. And next month Thomas Donilon, Barack Obama's national security adviser, is to visit. North Korea does seem one issue over which they might find common ground with their hosts. Both America and China accept that sooner or later they will have to engage the North Korean regime in some sort of dialogue. So both have an interest in talking North Korea down from the suicidal ledge on which its blood-curdling propaganda has stranded it. North Korea may have to be allowed to declare victory. This looks easy. Since, to its own people, it is a heroic nation threatened by imminent American nuclear onslaught, it can win merely by not being attacked. But North Korea has already spurned one chance to celebrate a triumph, after America on April 6th announced that, in deference to North Korean paranoia, it was postponing a routine missile test in California. Instead, North Korea stepped up its warlike rants. America and China both hope it will exhaust itself, perhaps with a final eruption, such as another banned test, to mark the 101st birthday of the dead but "eternal" president, grandpa Kim Il Sung, on April 15th. Certainly, America and China seem closer over North Korea than over the other two disputes most threatening East Asian stability. In China's stand-off with Japan over the Senkaku or Diaoyu islands, America officially takes no position on sovereignty over the uninhabited specks. But it sees them as under Japanese administration and hence covered by its security treaty with Japan. China resents this, arguing it encourages Japanese intransigence. Similarly, it blames America for stirring up trouble in the South China Sea by emboldening the Philippines and Vietnam to press territorial claims that conflict with China's. Here China shows no hint of flexibility. It has just announced that it is to open the Paracel islands, from which it expelled Vietnam in 1974, to tourists. Last month its navy sent a patrol to the southernmost tip of the vast area claimed in the South China Sea where Brunei and Malaysia also have claims. A juvenile delinquent, but he's our juvenile delinquent In fact, on North Korea, too, China is offering few signs of a fundamental shift in its stance. Short-term interests converge. Neither America nor China wants a war. Both seem to regard Kim Jong Un as an unhinged teenager who needs to be back on his meds. But in the long run, America worries more about a nuclear-armed North Korea. China worries about the country's collapse. An article this month in Global Times, a Communist Party newspaper, by Zhu Zhangping, an "independent observer," noted the danger North Korea's nuclear tests - just over 100km (60 miles) from the Chinese border - pose to China's water supply and the safety of its food. But, he argued, "North Korea still acts as a buffer." Were it to collapse, American troops would be at China's frontiers, and hordes of refugees might flood across the border. China has to "ensure the Kim regime's survival." China's leaders seem to agree. After Deng Yuwen, a deputy editor of a Communist Party journal, took to the pages of the Financial Times, part-owner of The Economist, to argue that "Beijing should give up on Pyongyang," he was suspended from his job. Those American senators have a point: the power supply that keeps North Korea's life-support machine ticking is the fuel, food, trade, aid and diplomatic protection that China provides. And China's leaders do not want to pull the plug.