“When you’re already $500bn DOWN, you can’t lose.” Donald Trump’s tweet earlier this week was a neat summary of the US president’s approach to trade: it combined economic illiteracy with the fallacy of an addicted gambler. Having set out plans to apply tit-for-tat tariffs to a collective $100bn worth of imports, the US and China now seem to be playing double or quits, with Mr Trump ordering officials to identify products on which to impose a further $100bn of tariffs, and Beijing declaring its intention to fight back.

If the two countries act on their threats, however, history suggests that both sides will lose — with unpredictable consequences that could go well beyond the economic fallout.

The most notorious example of self-defeating protectionism is the Smoot-Hawley Tariff Act of 1930, which raised US tariffs on more than 20,000 goods. After Canada and other trading partners retaliated, US exports fell by more than 60 per cent. Economists dispute the extent of the damage, but there is little doubt that the measures deepened the Great Depression and had lasting effects on diplomacy as well as trade — prompting countries that had shunned the Soviet Union to forge ties.