The Yen War: When Uncle Trump Joined Tokyo in the Ring

The conflict involving Iran and the broader Middle East added a layer of pressure. Japan is heavily dependent on imported energy, with roughly 95% of its crude oil imports coming from the Middle East. That makes Japan particularly vulnerable to disruptions around the region and the Strait of Hormuz.

Think of Japan as a household that imports almost all of its fuel. Now imagine the price of fuel suddenly shoots higher and the currency you use to pay for it is falling. That's basically Japan's situation. Higher oil prices mean Japan needs more dollars to pay for energy imports. More demand for dollars + more selling pressure on Yen = Yen gets weaker. And the weaker Yen then makes those already-expensive imports even more expensive. A rather unpleasant feedback loop.

This is probably the bigger structural story. Japan has spent years operating with exceptionally low interest rates, while U.S. rates have remained significantly higher. This creates the perfect playground for the famous Yen carry trade.

The basic idea is simple: Borrow cheaply in Yen → convert to dollars → invest in higher-yielding U.S. assets. Why borrow Yen? Because it is cheap. Why buy dollars? Because dollar assets offer higher returns. The result? Investors sell Yen and buy dollars. Do that at scale and the Yen gets pushed lower. It's basically the financial equivalent of everyone leaving one party because the drinks are free somewhere else.

The Middle East shock isn't only pushing up oil prices. It is also creating concerns about global inflation. And as inflation stays elevated, expectations for U.S. monetary policy has shifted from: "When will the Fed cut?" to "Wait… are we talking about rate hikes again?" Higher-for-longer U.S. rates make dollar assets even more attractive. And when the dollar becomes more attractive, the Yen has another problem.

So why is Japan worried about a weak Yen? At first glance, a weak Yen isn't necessarily bad for Japan. In fact, there are winners. Japanese exporters receive more Yen when they convert their foreign earnings back home. Foreign tourists also find Japan cheaper. That has helped make Japan an increasingly attractive destination for international visitors.

But there's a catch. Japan imports a lot of stuff — energy, raw materials, food, industrial inputs, etc. And when the Yen weakens, all those dollar-priced imports become more expensive. The same currency depreciation that gives exporters a boost can squeeze households and domestic businesses through higher costs. It's a classic case of "Congratulations, your exporters are winning. Unfortunately, your grocery bill is also winning."

And then there is Donald Trump. President Trump has repeatedly criticised what he sees as currency practices that give Japan an unfair trade advantage, particularly for Japanese manufacturers. The issue has also featured in broader U.S.-Japan trade discussions. So, Washington isn't exactly looking at the Yen from a purely charitable perspective.

There are trade considerations, there are inflation considerations and then there is something even bigger: the bond market.

This is where things get interesting. Japan's intervention was not merely a strongly worded statement saying "Dear Yen, please behave." They went into the market and bought Yen, selling foreign currency reserves to support it. Japan's intervention in late July was enormous, with some estimates suggesting around $53 billion — slightly more than Nigeria's entire FX reserves — may have been deployed. Earlier in April, Japan had already carried out a record-breaking single-day intervention of about $40 billion.

America joined the party. The U.S. Treasury directed the New York Fed to sell euros and buy Yen. Why euros? Because Washington wanted to support the Yen without selling dollars and thereby deliberately weakening the U.S. currency. In other words: "We want to help Japan… but don't touch the dollar." The operation marked the first coordinated U.S.-Japan yen-buying intervention since 1998. And yes, Treasury Secretary Scott Bessent even appeared to have a handwritten note saying: "Buy Japanese Yen $5–10bn."

Imagine having "buy $10bn of Yen" sitting on your to-do list next to "reply to emails." Central banking is a different kind of corporate life. But the big question is... Why did the US help? Is this just some charitable exercise, with the US being a "big brother to the rescue"? Well… Not when the side effects start spilling into the Treasury market.

Japan is one of the world's biggest holders of U.S. Treasuries — up to $1.1tn. If Tokyo needs to defend the Yen, it can potentially liquidate foreign assets including U.S. Treasuries to raise the foreign currency needed for intervention. Japan's foreign-security holdings did fall sharply following its earlier intervention, with analysts pointing to Treasury sales as one possible source of funding.

And here's the problem. Large-scale Treasury selling → Treasury prices fall → yields rise. Higher Treasury yields mean higher borrowing costs across the U.S. economy. Mortgages, corporate borrowing and government financing. Basically, the world's benchmark bond market starts catching a stray bullet from the Yen.

Why should global investors care? Because this changes the psychology of the Yen trade. For months, investors have been betting on continued Yen weakness. But intervention changes the risk-reward equation. If traders push USD/JPY aggressively higher, they now must consider the possibility that Japan and potentially the US could step in again.

That doesn't mean the Yen suddenly has a permanent floor. But it does mean the downside risk of betting against the Yen has become much more expensive. And when a trade becomes crowded, all it takes is one sharp reversal to send everyone running for the same exit. We saw exactly that dynamic after Japan's intervention earlier this year, when bearish Yen positions were reduced as intervention risk increased.

The intervention is a big deal, not because governments have discovered some magical cheat code for the foreign-exchange market. They haven't. You can fight the market, you can scare speculators, you can temporarily change the price. But you cannot permanently repeal economics.

And that is the biggest question hanging over the Yen. As long as the interest-rate differential between the U.S. and Japan remains wide, the structural incentive to borrow Yen and buy higher-yielding assets elsewhere remains. Intervention can change the price, change positioning, change market psychology. But unless the underlying fundamentals change, it may struggle to change the long-term trend.

And that is why the next move from the Bank of Japan matters just as much as the next move from the intervention desks. For now, though, the message from Washington and Tokyo is crystal clear:

"If you want to short the Yen, be my guest."