One great trading nation can pursue mercantilism and hope to get away with it. When two do so, deadlock results. The problem is illustrated by the latest unreliable firework ignited by the US in its trade war with China. It has banned chipmaker Fujian Jinhua from buying technology from US companies on national security grounds.

The justification is as strained as smiles at any meeting of American and Chinese diplomats. The US does not accuse Fujian of spying. Its explicit worry is that chips made by the business could eventually sell well enough to damage the finances of US rivals. Some have US military clients.

Seen in isolation, the ban looks like a salvo against free markets. The context is China’s push to catch up with developed nations in a range of tech-focused disciplines. These include robotics, electric power equipment and of course integrated circuits.