Japan and the US Spent Up to $85 Billion Defending the Yen in July. The Dollar Index Hit an 18-Month High Anyway.

The US Dollar Index broke above 102 on October 2 for the first time since April 2025, closing out eighteen months spent bouncing inside a narrow band between roughly 95 and 102. Dollar-yen was trading close to 158 the same week, a few yen short of the exact level that pushed Japan and the United States into a rare joint currency intervention back in July.

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What Changed

Treasury yields are doing most of the work here. The 10-year note climbed to around 5.33% this week, a level the market hasn't traded at since 2002, having pushed past its 2007 peak along the way, and that widening gap against Japanese yields is what has been pulling capital toward the dollar and away from the yen most of this year. That move echoes the dollar and the 10-year yield hitting multi-year highs together just a day earlier, and it comes as Japan's own government bond yields push to three-decade highs of their own. Neither move has done much to support the yen.

Why It Matters

Go back to July 31. The yen had just slid to roughly 164 per dollar, a 40-year low, on the same day the Bank of Japan held its policy rate at 1%, its highest level since 1995, rather than deliver the hike some investors had expected. Japan's Ministry of Finance spent an estimated 8.45 trillion yen, something in the range of $53 billion to $59 billion, buying its own currency in the open market. The US Treasury joined in with an amount it has never fully disclosed, estimated at $5 billion to $26 billion. Combined, the two governments may have spent as much as $85 billion defending a single exchange rate.

It worked, for about ten days. The yen jumped more than 1% to 155.20 per dollar, its strongest level since May, then held near 156.92 by August 3. Trump called the move "a signal of friendship," adding that "we're always there for Japan." Treasury Secretary Scott Bessent said the action "countered disorderly yen movements" and backed "Japan's decisive market and monetary steps." Finance Minister Satsuki Katayama promised Japan "will not hesitate to conduct further coordinated interventions."

Then the rate differential reasserted itself. By August 10 and 11, the yen had slid back to 159.09, erasing roughly half of whatever the intervention had bought. The problem analysts flagged at the time has not gone away: a one-time purchase of foreign currency cannot offset a persistent gap in interest rates, and it certainly cannot offset Japan's debt-to-GDP ratio, which sits above 200% and limits how aggressively the Bank of Japan can tighten policy without raising its own borrowing costs in the process.

The currency fight went diplomatic in late September. Trump met Prime Minister Sanae Takaichi on the sidelines of the UN General Assembly on September 25, and according to Japanese officials, he raised the weak yen directly, framing it as a drag on US trade competitiveness, a complaint that inverts the usual script, since a cheap currency typically draws criticism from the country living with it rather than from its trading partner. Takaichi agreed with the premise, telling reporters that "the yen's undervaluation poses a problem." Japan's 10-year government bond yield jumped to a 30-year high of 3.115% that same week.

On October 1, Takaichi went further, and arguably stepped back from intervention as her primary tool. "Our economic policy is not aimed at manipulating exchange rates," she said, laying out a different plan instead: "my administration aims to boost Japan's growth potential by increasing the economy's supply capacity through bold investment in crisis management and growth areas." That is a bet that structural reform, not another round of yen-buying, eventually closes the gap. It is also, in Takaichi's own framing, something close to an admission that the Bank of Japan's rate hikes, including the one last month, have not been enough on their own to prop up the currency.

What To Watch Next

Two numbers matter most from here. The first pair is 160 and then 164: how close dollar-yen gets to the levels that triggered July's intervention, and whether the Ministry of Finance has the appetite to spend tens of billions of dollars defending them again, or whether verbal warnings are all Tokyo wants to try this time. The second is the Bank of Japan's policy meeting later this month, where officials face a familiar bind. Hike rates to narrow the gap with the US and risk choking off growth while raising the government's own debt-service costs, or hold steady and watch the yen drift back toward the trigger zone.

There's a tension worth sitting with. Takaichi's growth agenda, heavy on fiscal spending for "crisis management," is the kind of program that tends to widen deficits before it narrows them, and a market already pricing in Japan's debt load may not reward that combination with a stronger currency. A weak yen isn't uniformly bad news either. It still pads the earnings of Japan's exporters, even as it raises the cost of everything the country imports, from energy to food. Both effects are real, and which one dominates probably depends on where the dollar index and the 10-year yield go next.

The Pulse24 Take

An $85 billion intervention buying ten days of relief isn't proof that currency intervention doesn't work. It's proof that it can't outrun a persistent interest-rate gap by itself, and investors spent the better part of two months finding that out the slow way. What's new as of October is that Japan's own leadership appears to agree. Betting on structural growth to fix a currency problem is a multi-year wager, not a trade, and the Bank of Japan, the Ministry of Finance, and the Federal Reserve all get a vote on whether it pays off before the next test of 160.