New Delhi, Aug. 10 -- Let us start with South Korea, because it is where the fault lines of AI mania are first becoming evident. Its Kospi fell by almost a third from its June peak as investors began to doubt that the world's AI build-out could be paid for. Then, on 31 July, it rose 17.9% in a single session-the largest one-day gain in the index's history-after the largest American cloud providers re-affirmed that they would keep spending. Hundreds of billions in market value vanished and returned within a fortnight.

Note what the Kospi actually is. Two firms, Samsung and SK Hynix, account for more than half its value and the bulk of its listed earnings. They make the memory that trains and runs advanced models. So the index did not mere...