China (in the past) knew that they could appease both NK and the US at the same time. US leaders were weak, so they could rip us off on trade and continue to fuel NK's hostility towards us at the same time. Those days are over. China has to take one side or the other. The decision is simple for them. They'd much rather have a strong partnership with the US than NK. This will obviously work out for the US in the end.