Reversing the yen’s long-term decline ultimately rests on Japan’s ability to take its real interest rates out of negative territory – without triggering market panic.

The U.S. wants to prevent Japan selling more U.S. bonds to stabilize the yen, which would send yields higher and interest rates up for U.S. consumers.

The US and Japan's first joint action on the yen in 15 years can only offer a temporary effect in defending the Asian currency, and a sustained reversal of the yen's long-term…