New Delhi, Feb. 25 -- Nothing has more starkly defined US President Donald Trump's second term than his aggressive and protean application of tariffs. The grounds range from remedying bilateral trade deficits (for the so-called reciprocal tariffs) and curbing engagement with hostile third countries (punitive levies for purchases of Russian oil) to pursuing geopolitical ambitions (the remarkable, if promptly retracted, threat to levy punitive tariffs on the EU for Denmark's refusal to cede Greenland).

The legal underpinning of his most sweeping actions was the International Emergency Economic Powers Act (IEEPA), a 1977 law that no previous president has used in such a fashion. Last Friday, the US Supreme Court ruled by a 6-3 majority that...