China is the only diplomatic obstacle standing in the way of a Second Korean War.
Failing that, Northeast Asia will likely enter a very destructive war. If North Korea does not change course with its development of an ICBM the United States will have no choice but to destroy their nuclear program with airstrikes. An American air campaign would have to be extensive in scale because of how numerous the North’s nuclear facilities are and their being shielded by air defenses. US airstrikes would almost certainly be met with a retaliatory North Korean ground campaign against the South. Their attack would open with a barrage from the thousands of North Korean artillery batteries aimed at the South. American and South Korean ground forces backed by American airpower would then return fire. North Korean artillery positions would be destroyed in about a week or two, but not until they have torn up much of South Korea.
Will China help prevent this?
In the past their aid has not been so forthcoming. But in the present, the incentive structures are now changed enough that China might deviate from its norm.
Until now China found North Korea useful so long as the North did not become so aggressive that America felt the risk of war was more acceptable than the risk of containment. As long as China could yank the chain of the Kim Dyansty and pull them back under a risk threshold America could, however warily, tolerate, the Chinese benefited by the continued existence of the regime. The division of Korea has served to divert extensive military resources owned by America and our Democratic allies in Japan and South Korea that might otherwise be eventually deployed against China.
A North Korea contained by America is obviously sound policy for the Chinese.
A war on the Korean Peninsula is not. The flood of refugees into China, disruption of commerce in Northeast Asia, and reunification of the North with the America-aligned South are all negative enough for China to want to avoid such a scenario materializing.
The new problem facing China is that containment, in any form, becomes an unacceptable strategic risk to America if the North Korea continues to pursue an ICBM capable of hitting America. As discussed before, an operational ICBM gives the North too far much geopolitical freedom to directly blackmail the United States. Because of this, the risk equation for the Trump administration has now transformed into one where the existence of the North Korean regime itself cannot be tolerated unless their ICBM program is terminated.
This new calculus also gives the Chinese not much less of an interest in using their leverage over North Korea, to a much greater extent than previous, to stop the program as the Trump administration has.
China is likely to pressure North Korea to desist. The question will then become whether whether the Hermit Kingdom will fold or dig in for a final confrontation. Given the mercurial nature of the Kims that question is not answerable until diplomatic options have been used up.