7th July 2026
The Japanese yen has fallen to levels against the US dollar not seen for around 40 years.
At first glance, that might sound like a problem only for Japan.
In reality, it has the potential to affect global financial markets, interest rates and even the cost of borrowing for the United States Government.
So why has one of the world's largest economies seen its currency weaken so dramatically?
The interest rate story
Currencies are influenced by many factors, but one of the biggest is interest rates.
Over the past few years, the United States has raised interest rates sharply to tackle inflation.
Japan has not.
While the US Federal Reserve pushed rates to levels above 5% during the inflation crisis before beginning to ease them, the Bank of Japan kept interest rates close to zero for much longer after decades of weak inflation and slow economic growth.
Investors naturally look for the best return.
If US government bonds are paying far more interest than Japanese bonds, many investors choose to move their money into dollars.
To do that, they sell yen and buy dollars.
As more people do the same thing, the dollar strengthens and the yen weakens.
Japan's long economic hangover
Japan has spent more than thirty years trying to escape economic stagnation.
Following the collapse of its property and stock market bubble in the early 1990s, the country experienced decades of weak growth and low inflation.
The Bank of Japan responded with extremely low interest rates and massive monetary stimulus.
Those policies helped keep borrowing costs low but also made Japanese assets less attractive to international investors once other countries began raising interest rates.
A weak currency isn't always bad
There is a tendency to assume a weaker currency is a sign of economic failure.
It isn't necessarily.
A weaker yen makes Japanese exports cheaper overseas.
Companies such as Toyota, Sony and Nintendo earn much of their revenue abroad.
When those foreign earnings are converted back into yen, profits increase.
That helps many large exporters.
The downside is that imported goods become more expensive.
Japan imports much of its energy and many raw materials, so households and businesses pay higher prices for fuel and imported products.
Why America should care
This is where the story becomes much more interesting.
Japan is not just another country.
It is one of the largest foreign holders of US Treasury bonds.
These bonds are effectively loans made to the US Government by investors around the world.
The United States depends on buyers continuing to purchase them in order to finance its budget deficits.
The currency dilemma facing Japanese investors
Imagine you are a Japanese pension fund.
You own billions of dollars' worth of US Treasury bonds.
Those bonds pay interest in dollars.
However, your pensioners expect to receive payments in yen.
Normally this works well.
But if the yen suddenly becomes extremely weak, or begins moving unpredictably, the calculation changes.
Some Japanese investors may decide they would rather bring money home.
To do that they would:
sell US Treasury bonds;
convert dollars back into yen;
invest more money in Japan instead.
Even a relatively small shift by Japanese institutional investors can involve hundreds of billions of dollars.
Why Treasury bonds matter
US Treasury bonds underpin much of the global financial system.
If large investors begin selling them:
bond prices fall;
bond yields rise;
borrowing becomes more expensive.
Higher Treasury yields affect almost everything.
Mortgage rates.
Business borrowing.
Government borrowing.
Consumer loans.
The cost of servicing America's enormous national debt.
If fewer foreign investors are willing to finance US borrowing, Washington may have to offer higher interest rates to attract buyers.
That increases the cost of running the federal government.
Is this already happening?
Not on a dramatic scale.
Japanese investors continue to hold enormous quantities of US government debt because Treasuries remain among the safest and most liquid investments in the world.
However, analysts are watching carefully.
As Japanese interest rates gradually rise after years near zero, investing at home becomes more attractive than it has been for decades.
That could slowly reduce demand for US government bonds.
The concern is less about a sudden financial crisis than a gradual change in global capital flows.
Why the yen matters to everyone
The yen is more than simply Japan's currency.
It influences investment decisions worth trillions of pounds and dollars.
For decades, Japan's ultra-low interest rates encouraged investors to borrow cheaply in yen and invest elsewhere—a strategy known as the carry trade. If Japanese rates continue to rise or the yen strengthens sharply, those trades can unwind quickly, causing volatility across global markets.
If Japanese investors increasingly choose domestic investments over American government debt, the consequences could extend far beyond Tokyo.
The United States may find financing its growing debt becomes more expensive.
Global interest rates could remain higher.
Financial markets could become more volatile.
The bigger picture
Japan's weak yen is not simply a currency story.
It reflects decades of economic policy, different approaches to inflation and interest rates, and the changing balance of the global financial system.
For years, America benefited from countries like Japan recycling their savings into US Treasury bonds, helping to keep borrowing costs relatively low.
If that pattern begins to change, the effects may be felt not just by governments, but by homeowners, businesses and investors around the world.
Sometimes the most important financial stories begin with a number on a currency exchange board.
The consequences, however, can reach every corner of the global economy.
Why Should the UK Care?
It is tempting to see this as a financial battle between Japan and the United States, but the consequences could reach Britain as well.
Financial markets are interconnected. If Japanese investors begin reducing their holdings of US Treasury bonds and American borrowing costs rise, the effects rarely stop at the US border. Higher US bond yields often push up borrowing costs around the world because American government debt is regarded as the benchmark against which many other investments are priced.
For the UK, that could mean higher costs for the Government when issuing its own debt, making it more expensive to fund public spending at a time when the national debt is already substantial. It could also influence the interest rates paid by businesses seeking investment and, ultimately, the rates offered to homeowners taking out mortgages or remortgaging.
British pension funds and investment managers also hold significant overseas assets, including US government bonds. Sharp movements in global bond markets can therefore affect pension fund performance and investment returns, even if the underlying cause originates thousands of miles away.
Perhaps the biggest lesson is that no major economy now operates in isolation. A decision by the Bank of Japan, a change in the value of the yen or a shift in Japanese investment strategy can ripple through global markets and eventually influence the cost of borrowing, investing and doing business here in the United Kingdom.
For many years, investors assumed America would always find willing buyers for its debt at relatively low interest rates. If countries such as Japan gradually begin investing more of their savings at home rather than financing US borrowing, the world may be entering a period in which governments everywhere—including the UK—face permanently higher borrowing costs. That would have implications not just for financial markets, but for future taxation, public spending and economic growth.