The US and Japan executed their first coordinated currency intervention in nearly 30 years last Friday, employing unprecedented trading tactics to trigger a sharp rebound in the yen. This rare cross-border monetary rescue operation, led by Trump's Treasury Secretary Bessent, not only broke multiple market conventions but also sparked widespread concern about potential ripple effects in the US Treasury market.

Japanese Finance Minister Mayumi Katayama confirmed this was the first coordinated yen-buying intervention by the two countries' foreign exchange authorities in 28 years. According to Bloomberg's analysis of Bank of Japan account data, Tokyo may have spent 8.45 trillion yen (approximately $54 billion) in independent intervention last Thursday, and an additional 5.33 trillion yen in the joint action on Friday. The yen had earlier touched 164, its weakest level since 1986, before the intervention boosted it to around 157.

The move shocked markets because the yen was gradually declining, not experiencing the chaotic crash that typically triggers joint intervention. The Treasury's strategy was also highly unusual, as it chose to sell euros rather than dollars to buy yen. Economists were surprised by Bessent's official intervention against interest rate fundamentals. The Bank of Japan's slow policy rate increase to 1% struggles to counter inflationary pressures, and global interest rate differentials will continue to be a long-term drag on the yen.

For Bessent, high-risk forex trading is nothing new. This former Soros Fund trader, who made his name shorting the British pound in 1992 and the yen in 2013, has taken a more aggressive interventionist approach since taking charge of the Treasury, last year leading a rescue of the Argentine peso. During a cabinet meeting last Friday, a photographer captured Bessent's to-do list with "Buy $5-10 billion yen" written on it. One major Treasury investor commented, "This isn't for soft power or the public good, so what is the US really doing?"

Wall Street institutions generally believe the core motivation for US intervention was to prevent Japan from selling massive amounts of US Treasuries to defend its currency, which could further destabilize US long-term borrowing costs. At the time, with the yen weakening and Japanese government bonds (JGBs) plummeting, markets clearly felt heavy selling pressure on US Treasuries. US long-term borrowing costs have already hit their highest since 2007, making them highly sensitive to any decline in demand. Since the early stages of the Iran conflict, foreign central banks' holdings of US Treasuries at the Fed have fallen to their lowest since 2012. A surge in Treasury yields could stimulate Japanese domestic investors to sell US Treasuries and repatriate funds.

On Monday, Japanese officials announced plans to use the Fed's repurchase agreement facility, allowing Tokyo to borrow dollars without selling US Treasuries. Brij Khurana, a portfolio manager at Wellington Investment Management, believes, "Bessent's view is that high US Treasury yields are due to high JGB yields." Khurana further analyzes, "Therefore, if you support the yen, you can lower Japan's yields, and consequently lower US Treasury yields." Mark Dowding, Chief Investment Officer for Fixed Income at RBC BlueBay Asset Management, also noted that the yen's decline "risks destabilizing" JGBs, "pushing up global long-term bond yields, to which the US is highly sensitive."

Beyond stabilizing US Treasuries, financial returns may also be a key consideration. Trump told media that "financial interests" were a motivating factor, emphasizing that the US "always supports Japan." Rushabh Amin, a multi-asset portfolio manager at Panoramic Global Investments, noted, "The US government wants a weaker dollar... and they also don't want investors to sell US Treasuries to protect their own currencies." Amin stated that traders shorting other currencies against the dollar are increasingly wary of "Bessent's buying." Although Bessent has previously supported the traditional "strong dollar policy," earlier times, senior US government officials have also frequently mentioned the impact of a strong dollar on domestic exporters.

The US Treasury declined to comment on the intervention. Katayama also declined to say whether US assistance was sought due to concerns about market contagion, nor would she evaluate the intervention's effectiveness. She reiterated, "This is the first coordinated yen-buying intervention by Japan and the US in 28 years. I have nothing further to add." Regarding US motives, Katayama noted that Bessent's statement "mentioned the word 'Abenomics'," adding, "My understanding is that the US side highly values Japan's strong economic measures, and I believe this is clearly reflected in their statement." Japan's Minister of State for Economic and Fiscal Policy, Minoru Kihara, similarly stated that exchange rate fluctuations have multiple impacts on the economy and prices, emphasizing close monitoring of forex movements for appropriate economic and fiscal policy management.

Analysts and traders are strongly warning that this currency boost could quickly reverse. Adam Posen, head of the Peterson Institute for International Economics and a Japan expert, criticized, "The irony is stunning that the person who worked for Soros and Stanley Druckenmiller in 1992 and broke the Bank of England now pretends that unilateral FX intervention alone can sustainably defend a currency." Chris Turner, Global Head of Markets at ING, commented, "The Treasury's arrival marks a new sheriff in town, warning speculators not to short the yen," signaling "the return of an era of forex activism." Market participants note that this action injects uncertainty into core currency markets, marking the full commencement of a new era of active government intervention to block trades not in the US's interest.

Kenneth Rogoff, a Harvard professor and former chief economist of the IMF, believes, "This is very clever and reminiscent of the rescue Bessent provided to Argentina, which was very successful." However, he cautioned, "Unless the US Treasury is willing to hold a massive amount of yen – which would be a complete radical shift – this is just a band-aid to buy the Bank of Japan some time." Analysts emphasize that fundamental bearish factors like surging oil prices, uncertainty over fiscal spending plans, and the Bank of Japan's relatively slow rate hikes cannot be resolved by a single intervention. Masahiko Loo, Senior Fixed Income Strategist at State Street in Tokyo, stated bluntly, "We all know intervention only buys time; the heavy lifting will fall on the Bank of Japan and Japan's fiscal policy."

For the US, the biggest risk is that coordinated intervention fails, and speculators once again target the yen or even the US Treasury market. Daleep Singh, Chief Global Economist at PGIM, issued a warning: "If the intervention fails to work, the spillover effects (on long-term US Treasuries) will be... extremely significant. So, buckle up."