New Delhi, Aug. 12 -- The scribble on a US Treasury notepad earlier this month might have looked casual, but it was no ordinary to-do list. Scott Bessent's note to buy tens of billion dollars' worth of Japanese yen was photographed and billboarded as a scoop, helpfully telegraphing the US treasury secretary's intention to prop up the Japanese yen.

Bessent worked for billionaire George Soros for 13 years, including as his chief investment officer between 2011 and 2015. Bessent's equivalent of holding a placard up for traders around the world worked in the short-term.

On cue, the yen rose against the US dollar after that intervention by the US Treasury, which used euros to buy yen.

But it was an unusual event that also cast an unflatteri...