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A rare joint intervention by the United States and Japan to support the yen is rippling through global financial markets, exposing vulnerabilities in portfolios worldwide, according to Nigel Green, CEO of deVere Group, one of the world’s largest independent financial advisory firms. His warning comes as Tokyo and Washington confirmed their first coordinated yen buying operation since 1998, a move that stunned traders and signalled deeper concerns about financial stability. 

The yen had plunged to around ¥164 per dollar, its weakest level in nearly four decades, before rebounding sharply toward ¥156 following the intervention. The U.S. Treasury, acting through the New York Federal Reserve, purchased yen alongside Japan’s Ministry of Finance, marking the first U.S. intervention involving outright yen purchases in almost 30 years. 

Japan may have spent as much as $36.6 billion to $59 billion defending the currency over two days of trading – last Thursday and Friday – according to Bank of Japan money market projections and central bank data cited by Reuters. The scale of the intervention underscores Tokyo’s determination to halt the yen’s slide, which has been exacerbated by widening interest rate differentials between Japan and the United States. 

Green argues that markets are misreading the episode as a simple currency defence. “When two of the world’s largest economies step into the market together for the first time in over a decade, they’re telling investors something about stress building beneath the surface of the global financial system,” he said.

He highlighted the central role of the Federal Reserve’s FIMA repo facility, which allows Japan to raise dollar liquidity without selling U.S. Treasuries. This detail, he said, reveals Washington’s concern that Tokyo might otherwise be forced to dump its massive Treasury holdings – larger than any other country’s – to fund currency defence, potentially pushing U.S. yields higher at a time of already elevated borrowing costs.

Reuters reporting supports this interpretation, noting that U.S. officials had been preparing for possible intervention since early 2026, with bilateral talks intensifying in May. Japanese Finance Minister Satsuki Katayama said she had held roughly ten rounds of discussions with U.S. Treasury Secretary Scott Bessent, reflecting unusually close coordination between the two governments. 

The coordinated action also comes amid rising pressure on Japanese government bond (JGB) markets. Higher JGB yields have historically spilt over into global borrowing costs, and a weak yen forces Japanese institutions to reassess their overseas yield exposure. Green warns that investors who focus only on domestic bond markets risk missing the extent to which global fixed-income dynamics have become interconnected.

He urged investors to stress test their currency exposure immediately. “Diversification across currencies, not just asset classes, is no longer optional,” he said, adding that gold and other traditional safe-haven assets are likely to benefit as volatility persists.

Bloomberg reporting has similarly noted that the New York Fed contacted major financial institutions ahead of the intervention, checking dollar-yen exchange rates – an early sign of U.S. involvement. The coordinated move sent the dollar tumbling roughly 1% against the yen, reinforcing expectations that authorities may intervene again if volatility continues. 

Green concluded that the yen’s sudden shift from “background noise” to a central market driver should serve as a wake-up call. “Investors who adapt their portfolios now, building genuine currency diversification and reducing overreliance on any one funding currency, will be far better positioned than those who wait for the next intervention to force their hand.”