This is not because Xi Jinping, China’s new leader, enjoys watching his 29 year-old neighbour, Kim Jung-un, brandish a toy that could obliterate Beijing as readily as Tokyo or Seoul. Tuesday’s nuclear test took the Disney-loving dictator closer to a miniaturised warhead that can be fitted to his ballistic missiles.

Rather, Beijing and Pyongyang are locked in a loveless dance of shared history, common enemies and domestic political and dynastic imperatives from which neither can easily escape. China protects and nourishes its wayward neighbour as if they were “as close as lips and teeth”—an old expression of Mao Zedong’s that has come back into vogue. But it has always been more a grimace than a smile.

North Korean officials routinely tear up investment contracts and walk away with Chinese money. Their soldiers regularly shoot Chinese traders across the Yalu River and treat Chinese fisherman to a style of hospitality that China sometimes shows to the Vietnamese.

“They abused the Chinese crew, smashed the boat and desecrated the Chinese national flag,” says a leading Korea expert, Zhang Liangui, referring to the “kidnapping” of 28 Chinese fishermen on May 28 last year.

“North Korea has always been an untrustworthy nation, China has given it so much aid, it really is a weird state,” says Zhang, who is a professor of international strategic research at the Communist Party School, from which Xi recently retired as president.

And yet the dance continues much as it began in 1950, when the direct boss of Xi Jinping’s father, Xi Zhongxun, lead the Chinese troops who saved Kim Il-Sung from annihilation in the Korean War.

New archive research in Moscow and Beijing shows how Kim convinced Stalin to let him invade the South, wrongly calculating that Truman would not intervene. It also shows how Stalin cornered [Stalin didn’t ask but left Mao no choice] Mao into stumping up the troops. Relations between the three god-like dictators never recovered and the dynamics of the Cold War were set in stone.

China lost 200,000 troops in pushing the American-led forces back down to the 38th parallel, including Mao’s favourite son. But those deceased Chinese soldiers do not feature in any North Korean museum, according to Chinese historian Shen Zhihua. China has been airbrushed from North Korean history to make room for the heroic and nation-defining deeds of Kim Il-sung. Airbrushing history, however, is something Chinese leaders can understand.

From China's side of the Yalu, the war was imposed on the Chinese people by "imperialist invaders,” as Xi put it in a 60th anniversary speech to veterans and troops in 2010. One of the history's most costly and pointless military stalemates was "a great victory in the pursuit of world peace and human progress," he said.

There is no trust or affection between Beijing and Pyongyang but they do have an alliance that was forged with "blood and steel," as veterans like to put it. The War to Resist American Aggression and Assist Korea is a central legitimising story for the Chinese Communist Party, the People’s Liberation Army and also their founding families, just like it is for the Korean Workers Party, the Korean People's Army and the dynasty of Kim. “The Chinese people would never forget the great contribution and sacrifice made by the nation's founders,” said Xi.

Although no serious scholar disputes that the war began with the North invading the South, it is also true that the Chinese army did not seriously intervene until the American-led forces had pushed right up towards the Chinese border.

Those old Cold War patterns of great power rivalry, existential fear and buffer states are re-emerging in more complex form today. Beijing is once again locked in a contest with Washington for regional influence, or domination, and Pyongyang is one of its only strategic friends. Xi, would like to demonstrate who has the upper hand in the relationship. He may even enjoy inflicting a modicum of pain. But the gentle tap on the wrist he gave his recalcitrant ally last night suggests - “all sides" should respond "calmly, through talks" - shows the underlying strategic calculus remains unchanged.

In any case, Chinese analysts are convinced that North Korea will not give up its nuclear program, it’s sole source of leverage, deterrence and self esteem, no matter what threats and incentives China might attempt.

“I don't think there will be much change in China's policy towards North Korea,” says Cai Jian, Professor of Korean studies at Shanghai’s prestigious Fudan University.

“As China grows, the United States adjusts its strategy towards East Asia to deter and encircle China,” he says. “What China needs is the survival and existence of the North Korean regime to help China maintain the regional balance of power.”

A version of this article will appear in Prospect magazine.