The phoney trade war of the last year is over. President Donald Trump has dropped a dangerously real cluster bomb on China.

“The first of many,” he growled with defiant satisfaction as he signed off on the most dramatic trade sanctions since the Smoot-Hawley Tariff Act of 1930.

The dispute centres as much on who will control the technology life-blood of the 21st century as it does on China’s $375bn (£265bn) trade surplus with the US – almost half of America's $800bn trade gap and patently "out of control" in Mr Trump's words. He says it must be reduced by $100bn immediately.

What makes this so menacing is that the world’s two dominant superpowers are in a state of escalating hostility over both...