Mr. Jang’s fall from power is the latest in a long succession of Chinese policy failures in North Korea. Many analysts argue that North Korea is a buffer state for China, used by Beijing to insulate it from American pressure. But the reverse is more the case: China is a buffer state for North Korea, for Beijing continues to be Pyongyang’s primary enabler.

Chinese foreign policy currently appears bold and assertive, but on the peninsula China’s stance remains exceedingly risk-averse. The question is why Beijing continues to exhibit such caution, even outright timidity, toward the North.

Some argue that the legacy of the Korean War weighs heavily on the minds of more traditional constituencies within the Chinese Communist Party and army. But deeper, current anxieties also inhibit Beijing. China fears that extreme actions by an unpredictable, heavily armed neighbor with a xenophobic leadership could trigger a larger crisis on the peninsula that would quickly involve China. Lacking realistic options to control North Korean behavior, China prefers instead to avoid doing anything that might alienate Pyongyang.

While China insists that it wants normal state-to-state relations with North Korea, it is not prepared to impose conditions on its isolated, troublesome neighbor, much less undertake a larger reassessment of its policies. As a consequence, Mr. Kim sees little reason to follow China’s advice, and he will continue to zealously guard against Chinese influence on the North.

The one exception to China’s passivity toward the North is its promotion of closer ties with Seoul. South Korea’s trade with China now exceeds $250 billion, more than the South’s combined trade with Japan and the United States. (China’s two-way trade with North Korea is worth approximately $6 billion.) South Korea’s president, Park Geun-hye, has already been welcomed on a state visit to Beijing; there has been no equivalent welcome for Mr. Kim, and it is not even certain that he seeks one.

China exhibits ample unease about addressing the larger risks posed by North Korea’s actions and goals. The purge of Mr. Jang serves only to underscore Beijing’s lack of influence in Pyongyang. The United States and South Korea both hope for more from China, but Beijing seems paralyzed in policy indecision.

The failures of policy on the Korean Peninsula are not those of China alone, but Beijing’s risk aversion bespeaks a larger absence of will and imagination on China’s part. Without a more candid conversation among Beijing, Seoul and Washington, the latent risk of an acute threat on the peninsula remains uncomfortably high.