The American-supplied missile defenses in South Korea, known as the Terminal High Altitude Area Defense system, or Thaad, has effectively killed any chance of China’s cooperating with the United States, they said.  “China is strongly opposed to North Korea’s nuclear weapons but at the same time opposes the defense system in South Korea,” said Cheng Xiaohe, an assistant professor of international relations at Renmin University. It is not clear which situation the Chinese leadership is more agitated about, he said.  Beijing interprets the Thaad deployment as another American effort to contain China. The system reinforces China’s view that its alliance with North Korea is an integral part of China’s strategic interests in Asia, with America’s treaty allies Japan and South Korea and tens of thousands of American troops close by, Mr. Shi said.  Washington insists the Thaad system, due to be installed in 2017, is intended to defend South Korea against North Korean missiles, and is not aimed at China. The system “does not change the strategic balance between the United States and China,” President Obama said after meeting with Mr. Xi in Hangzhou, China, a week ago.  But China is not persuaded. Chinese officials argue that the Thaad radar can detect Chinese missiles on the mainland, undermining its nuclear deterrent.  So despite what Chinese analysts describe as the government’s distaste for Mr. Kim and his unpredictable behavior, China’s basic calculus on North Korea remains firm.  Mr. Xi would continue to ensure that North Korea remained stable. The Chinese leader, 63, has shown disdain for the much younger Mr. Kim, 32. He has not invited him to China, and has authorized only sporadic visits by Chinese officials to Pyongyang. The two militaries remain largely uninvolved with each other.  But the personal and professional antagonisms do not alter Beijing’s goal of preventing a unification of North and South Korea under an American defense arrangement.  The longstanding fear that punitive economic action would destabilize North Korea makes it very unlikely that Beijing would cooperate with the United States on more stringent sanctions at the United Nations, according to Chinese analysts.  In March, after considerable hesitation, China agreed to Washington’s appeals and signed on to tough United Nations sanctions that included a ban on the export of North Korean coal.  Now as the West moves toward another round of United Nations sanctions, China’s mood is very different, said a former senior Chinese official who worked on North Korea. He said some officials were wondering why China would work with the United States at the United Nations after Washington went ahead with the antimissile system against Chinese wishes.  Meanwhile, the sanctions imposed in March have been enforced in only a desultory fashion, trade experts said.  A loophole in the sanctions allows North Korean coal to be sold if the proceeds go for humanitarian reasons, and that opening seems to have been exploited, said Stephan Haggard, a Korea expert at the University of California, San Diego.  The sale of coal since the sanctions came into force was down 12 percent from the same period last year, a marginal amount, he said.  On its own, the United States imposed so-called secondary sanctions on business entities that do business with North Korea and in the United States. But North Korean businesses have found Chinese partners or are using front companies to use smaller Chinese provincial and municipal banks, Mr. Haggard said.  There are differing opinions in China about whether an oil embargo — an unlikely punishment — would result in Mr. Kim’s giving up his weapons.  If China stopped the flow of oil, North Korea would face a severe economic crisis in about one year, and then face a choice between keeping its economy going or compromising on its nuclear program, the former senior official said.  It is possible that at that point Mr. Kim would come to the negotiating table, the former official said.  But others disagree, saying the Chinese government would not dare cut the oil supply, knowing that North Korea would be able to get supplies from Russia and elsewhere.  “The fundamental reason for not cutting oil is they don’t want to sacrifice the buffer zone, and they also know if they cut off the oil supply it will not force Kim Jong-un to surrender his weapons,” Mr. Shi said.  Mr. Shi questioned why China would want to risk making North Korea into an enemy by cutting off the oil supply. “If you cut off the oil, there is a 50 percent possibility North Korea will not surrender their weapons, and they will hate China even more,” he said.  China’s continued support of North Korea is a fundamental reason the United States should stop relying on China for progress on reducing the North Korean nuclear threat, said Joel Wit, a visiting scholar at the U.S.-Korea Institute at the School for Advanced International Studies at Johns Hopkins University. Mr. Wit is among a group of American North Korea experts who believe the United States should take the lead and negotiate with the North.  An earlier version of this article misstated the name of a visiting scholar at the U.S.-Korean Institute at the School for Advanced International Studies at Johns Hopkins University. He is Joel Wit, not Joel de Wit.
Pyongyang’s aggression is an embarrassment for Beijing, but a collapsed North Korean regime worries Chinese leaders much more, analysts say.