Why the Historic US-Japan Intervention Has Failed to Halt the Yen's Slide

The Japanese yen has erased about half the gains from a historic US-Japan intervention less than two weeks ago, trading above 159 per dollar and approaching the psychologically important 160 level. The reversal came despite Washington joining Tokyo to buy yen and pledges from both sides that further action could follow.

Japan's currency strengthened to 155 in the days after the intervention, after it crossed 163. But the fundamental forces that have pressured the yen to multi-decade lows have proven resilient against short-term measures.

Carry Trades Reassert Themselves

“Intervention has scared markets, but has not stopped the laws of finance which say money flows in the direction of maximum returns … as long as the cost of money in Japan is lower than the return overseas, carry trades will re-assert,” said Jesper Koll, expert director at Monex Group.

At the heart of the problem is the gap in returns between Japan and the US. Japanese borrowing costs remain far below those in the US and other markets, prompting investors to borrow cheaply in yen and invest in higher-yielding assets, the classic carry trade. Higher Treasury yields and elevated oil prices, which pose a particular problem for energy-importing Japan, have also restored macro forces favouring the dollar.

Not everyone sees the intervention as a failure. “Intervention has succeeded in reducing speculative excess and raising the risks for traders betting against the yen, even if it has not eliminated the underlying yield advantage supporting the dollar,” Koll said.

Yield Gap Remains Wide

Masahiko Loo, senior fixed income and currency strategist at State Street Global Advisors, said the intervention successfully reset market psychology and demonstrated an unusually strong degree of US-Japan policy coordination. “What it has not yet done is eliminate the yield advantage supporting the dollar.”

The yield gap remains wide. Benchmark 10-year US Treasury yield stands at 4.686%, compared with 2.846% for 10-year Japanese government bonds, leaving investors a substantial incentive for holding US debt. “It’s better understood as a success in slowing speculation but not yet a success in changing fundamentals,” Loo said.

Attention now turns to the Bank of Japan, whose next monetary policy meeting is scheduled for September. Koll said the bigger shock for investors was not the intervention itself but the BOJ’s reluctance to tighten policy more aggressively. He said that raises questions over whether concerns about the banking system or Japan’s enormous public debt burden are constraining policymakers.

Rate Hikes Alone May Not Be Enough

In the absence of higher Japanese rates or falling US yields, investors still have an incentive to send money overseas. John Wood, chief investment officer for Asia at Lombard Odier, said the latest intervention would probably have “a limited time effect,” arguing the BOJ might need at least two more rate increases to draw a line under the currency’s weakness.

But interest rates may only be part of the explanation. Crédit Agricole CIB says the deeper problem is an “asymmetry of investment power” between the two economies. Massive US investment in artificial intelligence and other projects continues to attract capital, while Prime Minister Sanae Takaichi’s planned public-private investment push has yet to fully materialize.

“What is needed to correct the weak yen is not interest rate hikes, but to expand investment,” it said. That suggests a sustainable yen recovery ultimately requires Japanese assets to become more attractive, encouraging domestic savings to remain at home rather than chase returns abroad.

For now, intervention might function less as a mechanism for reversing the yen’s decline than as a guardrail against its acceleration. Loo said the 160 level has become “a political line in the sand,” meaning another rapid move through that threshold could draw officials back into the market.

“I would not rule out another intervention, particularly if the move becomes rapid or disorderly,” he said. “Ultimately, though, intervention can buy time, but the heavy lifting will fall on BOJ normalization as early as September.”

Washington and Tokyo have also sought to strengthen that deterrent by highlighting the Federal Reserve’s foreign and international monetary authorities’ repo facility, which can provide dollar liquidity against Treasury securities and reduce the need for Japan to sell its US bond holdings to finance intervention. Treasury Secretary Scott Bessent has signaled support for expanding the backstop. That makes it potentially more expensive to keep betting against the yen, but it doesn’t make the underlying trade disappear.

“Scaring markets is easy, getting markets to follow needs changed incentives and trust,” said Koll.