US President Donald Trump wants China to solve his number one foreign policy challenge - taming the nuclear ambitions of North Korea's mad young dictator, Kim Jong-un.

But China would like to use North Korea to weaken the US-South Korea alliance and undermine the US's position in Asia.

North Korea is a very curious case. During the Cold War period, it was not very different from other communist countries. It had a centrally planned economy which seemed to work well for a while, before its inherent inefficiencies eventually became evident.