In a somewhat public attempt to mend fences, North Korean leader Kim Jong-un has dispatched Choe Ryong-hae, vice chairman of the country's top military body, to explain North Korea’s recent actions. Since the 30-year old North Korean leader took power in Dec 2011, he has carried out two long-range rocket launches and a nuclear weapons test. This has presumably bristled the Chinese, who regard nuclear stability on the Korean peninsula as a top priority. A nuclear–capable North Korea would trigger East Asian nuclearization, forcing China to shift its focus on military modernization to nuclear balancing against erstwhile rivals such as Japan and South Korea.
Reports hint of a growing rift between Pyongyang and Beijing. Choe received a lukewarm reception from the Chinese; the letter he presented from Kim to Xi was reported as "terse" by a source close to Pyongyang and Beijing. Choe tried to justify the recent sabre-rattling and troop mobilization as necessary for Kim Jong-Un's power consolidation, a move designed to weed out disloyal elements in the military. Yet Choe's overtures were met with the usual Chinese refrain of wanting "calm and restraint" on the Korean peninsula, and urges to put the economy, not nuclear weapons, first.
It is commonly known that Beijing is Pyongyang's indispensible ally. Pyongyang relies on China for more than 70% of its external trade. China serves as the conduit for foreign investment finding its way into North Korea. China is the only country whose sanctions to North Korea would really bite.
But the reverse is also true. Pyongyang is a key strategic ally of China. North Korea is an important buffer state against U.S. military presence in the South. China does not really have many staunch allies, to which North Korea is an exception. Also, regime change would tremendously increase the economic strain North Korean refugees put on Chinese resources and create power vacuums which could result in further extensions of U.S. influence in the Korean peninsula.
China's optimal outcome would be a negotiated solution that denuclearizes the Korean peninsula, guarantees the regime survival of its only reliable ally, and creates further opportunities for the expansion of Chinese state capitalism. What this means is that despite their differences, and despite recent Chinese moves that seem to ally with U.S. sanctions policy on North Korea, Chinese interests dictate that they would always be the backer of last resort for their North Korean allies.