Every intervention to prop up the yen is also creating a fresh opportunity to sell it.

The historic joint U.S.-Japan action last month has done little to shift the forces weighing on the currency, with the yen sliding back toward ¥160 per dollar less than two weeks later.

The key is the wide gap between interest rates in Japan and elsewhere. Investors can borrow the low-yielding yen and use the money to buy higher-yielding assets — a strategy known as the carry trade.