New Delhi, March 23 -- Some of Donald Trump's reasons for imposing tariffs on countries are understandable, even if their fitness for the purpose is debatable. Demanding that Mexico impose tighter border controls, Europe raise defence expenditure and China prevent fentanyl smuggling or else suffer economic punishment is coercive diplomacy. A good threat comprises a promise of punishment if the demand is not met and, importantly, non-punishment if it is conceded. If Washington proceeds to impose tariffs on Mexico, Europe and China despite their concessions, it will not be coercive diplomacy. It will be something else.

Other reasons are dogmatic. Trump has been clear that Europe, Canada, China and the rest of the world have been unfairly b...