In 1984, China agreed a ‘one country, two systems’ treaty with the UK, designed to control the relationship between Hong Kong and China for 50 years after Britain ceded control of the colony in 1997. It has now broken the treaty, for no other reason than that it can. So what next in that tinderbox?

Classical Athens was at war three years out of four, and if arbitration had failed to solve the treaty problem, it would have gone to war.