The entry of North Korean troops risks a dangerous escalation in the Russia-Ukraine conflict. It also puts Beijing in a tight spot

In October 1950, barely a year after the Chinese civil war ended, Mao Zedong sent the first Chinese soldiers to fight in the Korean war. Between 180,000 and 400,000 of Chairman Mao’s troops would die in that conflict, including his own son. But it was important to defend North Korea in that battle, Mao reportedly said, because “without the lips, the teeth are cold”.

That Chinese idiom has been used to described China and North Korea’s close relationship for more than seven decades. China sees North Korea as a strategic security buffer in the region, while North Korea relies on its superpower neighbour for economic, political and military support. But that relationship is now under strain thanks to another war which is drawing Communist-rooted countries into a common battle.