The friendship seems to be wearing thin. Every time North Korea makes a belligerent provocation in East Asia, the world waits, breathless to see what China will do. More often than not, Beijing does nothing. But there are signs that it is as exasperated with Pyongyang as the rest of its neighbours.
China is the only country that North Korea's isolated leadership regularly consults. In October, high-level Chinese diplomats attended a military parade in Pyongyang, appearing to endorse the shaky succession of Kim Jong-un. Beyond that, diplomats have been tight-lipped on relations.
The Kim dynasty's original patrons in Moscow have long since cemented strong economic ties with Seoul at the expense of relations with Pyongyang. The same logic would seem to apply to Beijing. North Korea's utility as a buffer state pales in comparison to trade relations with the South in the changing East Asian landscape.
That seems to be borne out by data in the WikiLeaks cables released last week. Senior Chinese officials reportedly told their South Korean and American counterparts that North Korea was behaving like a "spoiled child" and that Beijing would welcome a unified peninsula governed by Seoul.
"Even though China seems to side with North Korea, they must be worried since their fundamental policy on the Korean peninsula is peace and stability," Lee Man-sop, the former chairman of the South Korean National Assembly, told PBC radio in Seoul yesterday.
For the South, the fundamental challenge is to bide its time, no easy task as the North shells civilians on Yeonpyeong island and stealthily attacks South Korean ships. But Seoul is long used to such provocations. From a commando raid on the presidential Blue House to a terrorist bombing of a civilian aircraft, South Korea has weathered the predations of its fratricidal neighbour for decades, all the while growing economically more powerful.
Lacking other strengths, Pyongyang's diplomacy relies on military brinkmanship. Internationally isolated and domestically weak, the regime is still dangerous. There may even be a lunatic fringe in the military that believes it can win a war. It cannot, but the toll would be terrible.
Seoul must talk a tough line after every attack, but it - and the rest of the world - should know that Pyongyang is its own worst enemy. Every day spent waiting is a victory for the vibrant South Korea, and a further defeat for the isolated North Korean regime.