The good news for Scott Bessent, the prickly US Treasury secretary, is that he has established control over one of the world’s most challenging financial markets, seemingly imposing his will against the odds where many before him have failed. The bad news is that this is not at home but in Japan.

Currency market specialists are still reeling from the news at the very end of July that the US had, under the direction of Bessent, stepped into Japan’s long-running battle with a worryingly weak yen. Bilateral currency market interventions are vanishingly rare, especially in chunky currencies such as this — the third most actively traded in the world.

But Bessent threw his weight behind the Japanese authorities, which had been buying yen to try to fluff up its value, with the Treasury selling euros, somewhat bizarrely, to buy some yen of its own. The amounts involved were quite small, somewhere in the order of $500mn — a large sum of money in real life but pocket change in the vast global currency markets. Still, what the intervention lacked in size, it delivered in shock factor.

Bessent has taken some heat over this highly unusual move including, perhaps predictably, from US senator Elizabeth Warren. The Treasury secretary gave her “sciolistic” letter short shrift. (I had to look it up too. It means superficial, or amateurish.) He offered to give her a tutorial in “Foreign Exchange for Dummies”.