On the balmy Saturday of May 13, 1950, a private train carrying North Korean leader Kim Il-sung arrived quietly in Beijing.

Kim, who grew up in China and joined the Chinese Communist Party (CCP) in 1931, had not visited Beijing since returning to Korea in 1945. He was on a secret mission to meet Mao Zedong ( 毛澤東 ) and inform the Chinese his decision to unite Korea by force.

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Unlike his meeting with Soviet leader Joseph Stalin a month earlier, Kim did not come to discuss the war plan or seek Mao’s approval. Kim “released no details of his military plan, let alone the date of the action”, according to the Chinese’ memo of the meeting. He simply told Mao that Stalin had approved his plan to attack the south. The Chinese leader listened stoically and then asked Kim what possible response he expected from the United States.

“The possibility of American intervention cannot be ruled out, but the war will be over in two to three weeks. It will be too quick for them to do anything,” said a confident Kim. When Mao offered three Chinese armies to help Pyongyang, Kim responded “arrogantly” (in Mao’s words) that the North Koreans could solve the problem by themselves.

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Kim’s move could not have come at a worse time for Beijing. Mao was a keen supporter of using force to spread communism to Korea and Vietnam, but he hoped to do so after finishing off the Chinese civil war and conquering Taiwan. Earlier that year, the Chinese leadership had demobilised 1.4 million ground troops so they could shift war resources to prepare for the Taiwan campaign. The outbreak of the Korean War suddenly changed priorities.

Given so much was at stake, the Chinese were eager to get first-hand intelligence. Yet Kim played his cards close to his chest, even after the war broke out on June 25. He formally notified Beijing about the invasion only on June 27 – by then the North Korean troops had marched into Seoul.

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In early August, the CCP central committee decided to send senior commander Deng Hua to assess the war situation in Korea. When Deng arrived in the border city of Andong, he was abruptly told by Pyongyang that the time was not right for his visit. Desperate for intelligence, Premier Zhou Enlai (周恩來) dispatched some lower-ranking military intelligence officers to Pyongyang, posted as members of the Chinese embassy.

Only when the American forces landed in Incheon in September, totally reversing the course of the war, did North Korea turn to China for help. Even then, Kim did not go to Beijing himself. Instead, he sent Pak Il-yu and Pak Hon-yong – the two leaders of the so-called Chinese section within the Korean Communist Party. Both were later purged by Kim.

China entered the Korean War after some painful deliberation. By the end of the war, the Chinese had lost between 200,000 and 400,000 lives.

The war is heatedly debated to this day. But one thing is clear: even at the height of their relationship, North Korea eyed Beijing with mistrust and suspicion.

This is perhaps not surprising. Mao, Kim and their Vietnamese counterpart Ho Chi Minh are in fact dyed-in-the-wool nationalists. Their belief in communism might be strong, but it was utilitarian by nature. To them, a Leninist-style revolution was the best path to achieve the ultimate goal of national liberation.

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While Kim and Ho often relied on China for assistance, they were sensitive to the old “Central Kingdom” worldview of Beijing. Korea, in particular, had been a tributary state to China long before modern times. Kim built his regime by fashioning himself as the liberator of Korea from foreign imperialist rule. He could not afford to let the new country be seen as a client state of China.

This became the psychocultural basis of the peculiar relationship between the two countries that remains today. Pyongyang may need China for many things, but it always stresses that it is a free agent in the international arena.

Kim Jong-un is largely following in his grandfather’s footsteps. When China offered Pyongyang nuclear protection in exchange for the latter giving up its own nuclear weapon programme, it was ironically reminiscent of what had happened between the Soviet Union and China decades ago.

Both Stalin and Nikita Khrushchev offered China nuclear protection. Instead of feeling reassured, a nationalistic Mao perceived this as a gesture to make China the “little brother” of Moscow. Despite China’s dire economic situation and international pressure, Mao insisted on developing China’s own nuclear bomb.

Chinese leaders are painfully aware of the delicate nature of dealing with a sensitive North Korea. Beijing often stresses that it has no “special influence” over Pyongyang. This is partly true as members of the “Chinese faction” in North Korea have been purged by Kim. But Chinese leaders are also aware that if they publicly claim to have leverage over North Korea, it will drive the nationalistic young leader farther away.

That is why Beijing views Washington and Tokyo’s open call for it to “rein in” North Korea with suspicion. The Chinese leadership construes this as an attempt to place blame on China for North Korea’s reckless actions. What’s more, such high-profile calls may drive a wedge between Beijing and Pyongyang. The louder Washington asks China to exert influence, the more eager Kim will be to show he is the master of his own destiny.

Chow Chung-yan is executive editor of the South China Morning Post, overseeing daily print and digital operations