Robert Rubin, the former U.S. Treasury secretary who added American firepower to the successful defense of the yen in the late 1990s, had some rules for foreign-exchange intervention. Such moves needed to be very rare and ought to be startling. The more surprise, the better, because a market whose value has swollen to $7.5 trillion a day could easily swamp such action.

As a former Goldman Sachs Group chief, Rubin also appreciated that currencies ultimately reflect the underlying conditions of an economy, relative to peers. Japan would do well to remember that as it debates how and when to battle a surging greenback.

Governments do have agency, but the objectives need to be clear. Trying to induce a sustained rally in the yen is a nonstarter as long as the gap in interest rates between the U.S. and Japan is so wide, with little prospect of a meaningful change for a considerable period. What officials can do is manage the yen's weakness, smooth out sharp drops, and make some gung-ho dealers think twice, at least on occasion. The more modest and plausible the ambition, the greater the chances of avoiding disappointment — and retaining credibility.