The 28-Year Rule: Why the US Just Intervened to Save the Yen

Abstract

On July 31, 2026, the US Treasury did something it had not done in 28 years: it bought Japanese yen. The yen had crashed to a 40-year low near 164 per dollar, and Japan's own attempt to defend it the day before had barely worked. Washington did not step in out of kindness. It stepped in because of a $1.13 trillion problem sitting on Japan's own books, one that threatened to push up borrowing costs across the entire US economy.

A Rule Broken Twice in Thirty Years

Think of currency intervention as an emergency brake. Under normal conditions, nobody touches it. US policy has worked this way since August 1995: officials believe exchange rates should be set by the market, not by governments, and they only step in when something looks broken. This stretch is often called the Era of Restraint.

That restraint shows up clearly in the numbers. Across every currency in the world, the US has intervened just four times since 1995. Only twice has it specifically bought yen to prop up its value: once during the 1998 Asian Financial Crisis, and now.

💡 Quick Explainer: These interventions are "sterilized," meaning they do not change US interest rates. When the New York Fed (the Federal Reserve's markets desk) buys yen, it makes a separate trade at home that puts an equal amount of cash back into US banks, canceling out the effect. Instead of moving rates directly, the operation works by sending a signal ("we think this price is broken") and by nudging investors to hold slightly more yen and slightly fewer dollars than before.

Why the Yen Kept Sliding

The reason is simple interest rate arithmetic. The Bank of Japan kept its policy rate near 1 percent while other major central banks kept theirs much higher. That gap turned the yen into what traders call a funding currency: investors borrow yen cheaply, convert it to dollars, and buy higher-yielding US assets, pocketing the difference. This is a carry trade, and every new one adds fresh selling pressure on the yen. It keeps working as long as the rate gap stays open.

Two Days, Two Attempts

Japan tried to fix this alone first. On Thursday, July 30, the Bank of Japan spent ¥8.45 trillion (between $53 billion and $59 billion) selling dollars to buy yen, its biggest single-day intervention on record. It barely mattered. Automated trading algorithms absorbed the cash within hours, and the yen slid back past 160 per dollar during US trading hours that same day.

So on Friday, July 31, US Treasury Secretary Scott Bessent ordered the New York Fed to buy between $5 billion and $10 billion worth of yen. The trade had a clever twist: instead of selling dollars directly, the Fed sold euros to buy yen. That way, the operation strengthened the yen without also weakening the dollar against everything else. Primary dealers, the big banks like Goldman Sachs and Morgan Stanley that trade directly with the Fed, executed the orders. By the end of the New York trading day, the yen had strengthened from 160.73 to 157.40 per dollar, a 3.3 percent move in a single session.

The Real Reason: Protecting the Bond Market

Here is the part that actually explains the urgency, and the part that matters most if you are not planning a trip to Tokyo. Japan owns more US government debt than any other country: over $1.13 trillion in Treasury bonds. Defending a currency costs real money, and if Japan had kept fighting the yen's slide alone, it would eventually have needed more dollars. The easiest way to raise them is to sell some of those Treasury bonds.

Markets do not wait around to find out if that actually happens; they price in the risk immediately. Just the fear of Japanese selling pushed the 30-year US Treasury yield toward 5.24 percent in late July. That number matters more than it sounds. Treasury yields set the floor for almost every other interest rate in the US economy: mortgages, car loans, credit cards, and even federal student loans, which are priced directly off the 10-year Treasury yield each year. When that yield climbs, borrowing gets more expensive for everyone, not just the US government.

The Fed has a tool built for exactly this situation: the FIMA Repo Facility. It lets foreign central banks borrow dollars by temporarily pledging their Treasury bonds as collateral, instead of selling them outright. Paired with the yen purchase, it let Japan defend its currency without dumping US debt onto the open market. Washington was not rescuing Tokyo out of generosity. It was making sure Japan's currency fire did not turn into an American bond market fire.

A Brake, Not a Fix

Because the operation is sterilized, it does not touch the actual problem. The Fed and the Bank of Japan still have very different interest rates, so the carry trade that dragged the yen down in the first place has not gone away, only paused. Pulling the emergency brake stops the immediate skid, but it does not explain why the car was speeding in the first place. Until the two central banks' rates actually move closer together, this kind of joint intervention will keep being the last resort for a problem that interest rate policy keeps recreating.