New Delhi, Aug. 3 -- For the first time in three decades, the United States has reached into a foreign currency market to defend an ally's money, and the story behind that decision reveals as much about anxieties in Washington as a crisis unfolding in Tokyo.

The clearest early signal came not from a statement but a photograph. At a cabinet meeting at Camp David on Friday, a Reuters photographer caught Treasury Secretary Scott Bessent's notepad over his shoulder. Under the underlined heading "To Do" sat one instruction: "Buy Japanese Yen (JPY) $5-10 bil."

President Donald Trump later confirmed the operation himself, framing it as an act of friendship rather than policy. "Japan's been very good to us, with the exception, of course, of Pea...