In a rare intervention, the US has propped up the Japanese yen. The move is seen as a bid to hold off a further rise in yields for US government bonds – amid Japan’s sell-off of US Treasuries to fund its shoring up of the yen – a rise that threatens the US artificial intelligence bubble. That the United States bought yen for the first time in decades – coordinating with Japan – had a bigger psychological impact on the market than Tokyo’s interventions alone. But give it three or four weeks and...