TOKYO — Japan’s foreign-exchange reserves recorded their largest monthly decline ever in August after authorities launched an unprecedented campaign to support the yen, highlighting the extraordinary scale of Tokyo’s effort to prevent further currency weakness.

Japan’s reserves fell by $79.6 billion, or 6.18%, to $1.208 trillion at the end of August, according to data released by the Ministry of Finance. The decline followed record dollar-selling and yen-buying intervention designed to stabilize the Japanese currency.

Tokyo Spent Nearly $100 Billion Defending the Yen

Japanese authorities spent approximately 15.4 trillion yen, equivalent to about $98.7 billion, on currency intervention between July 30 and August 26.

It was the largest amount Japan has ever deployed in a single month to influence the foreign-exchange market. The intervention came after the yen fell toward levels that policymakers considered increasingly problematic for the economy.

The operation involved selling U.S. dollars and purchasing yen, effectively increasing demand for Japan’s currency.

The move helped push the yen away from levels close to 164 per dollar, a roughly 40-year low, toward the mid-150s.

Foreign Securities Took the Biggest Hit

The largest component of the reserve decline came from Japan’s foreign securities holdings.

The country holds a substantial portion of its reserves in overseas securities, with U.S. Treasury securities believed to account for a significant share. Foreign securities fell by approximately $87.8 billion during August, according to Finance Ministry data.

The sharp reduction has led market participants to conclude that Tokyo likely sold part of its foreign-asset holdings to help finance the intervention.

However, the reserve data do not provide a complete breakdown of which individual securities were sold.

Rare Cooperation With the United States

The currency operation also carried an important international dimension.

Part of the intervention was conducted jointly with the United States, marking the first coordinated yen-support operation between Tokyo and Washington since 2011. The cooperation surprised financial markets because investors had not widely expected such direct intervention.

The coordination is significant because Japan is one of the world’s largest holders of U.S. government debt.

Large-scale Japanese sales of Treasury securities can potentially affect U.S. bond markets, making the way Tokyo manages its reserves an issue of interest not only for Japan but also for Washington and global investors.

Japan Still Has a Large Financial Buffer

Despite the record decline, Japan continues to hold one of the world’s largest pools of foreign-exchange reserves.

The remaining reserve stock of around $1.2 trillion gives Japanese authorities substantial resources if additional intervention becomes necessary.

However, repeated interventions on the same scale could become increasingly expensive.

Currency intervention can influence exchange rates temporarily, but it does not eliminate the underlying economic forces that cause a currency to weaken.

Why the Yen Has Been Under Pressure

The yen has faced pressure from differences between Japanese and U.S. interest rates, expectations for monetary policy and broader movements in global financial markets.

Japan has also faced higher import costs when the yen weakens because energy, raw materials and other goods purchased overseas become more expensive.

That creates a difficult policy balance for Japanese authorities.

A weaker yen can support exporters by making their products more competitive internationally, but excessive depreciation can increase household and business costs through higher import prices.

Markets Are Watching the Bank of Japan

The scale of intervention could increase pressure on the Bank of Japan to consider monetary policy as another tool for supporting the currency.

Market expectations for a Japanese rate increase have already strengthened. Investors are examining whether higher Japanese interest rates could reduce the gap between returns available in Japan and those available on dollar-denominated assets.

The yen has also been influenced by expectations for U.S. Federal Reserve policy. Investors are currently assessing whether the Fed could raise rates again after stronger-than-expected U.S. employment data.

That makes the direction of both Japanese and U.S. monetary policy particularly important for the currency.

Tokyo Has Another Option

Japan and the United States have indicated that Tokyo could potentially use a Federal Reserve liquidity facility rather than relying exclusively on outright sales of U.S. Treasury securities during future interventions.

Such a mechanism could provide Japan with access to dollar liquidity while reducing the need to dispose of large quantities of Treasury assets.

That option could become important if currency pressure returns and Japanese officials want to preserve their reserve portfolio.

Intervention Cannot Solve Every Problem

Currency intervention can be powerful when markets believe policymakers have the determination and resources to act.

But analysts generally distinguish between temporarily influencing exchange rates and changing the fundamental reasons behind currency movements.

If investors continue to expect large differences between Japanese and foreign interest rates, or if confidence in Japan’s economic outlook changes, intervention may have only a temporary effect.

That is why monetary policy, economic growth and fiscal conditions remain crucial alongside foreign-exchange operations.

A Warning for Global Bond Markets

Japan’s intervention has implications beyond the yen.

Because Japanese institutions and government authorities hold enormous quantities of overseas assets, changes in Tokyo’s reserve strategy can influence international bond markets.

The possibility of further Treasury sales is particularly important at a time when global investors are already monitoring government borrowing costs and rising bond yields.

Even modest changes in Japanese demand for foreign bonds can have consequences when global fixed-income markets are under pressure.

What Comes Next

Japan’s record intervention has demonstrated that authorities are willing to use substantial financial resources to defend the yen.

But the unprecedented decline in foreign reserves also shows the cost of that strategy.

With the yen still sensitive to U.S. interest-rate expectations and global energy prices, investors will closely monitor whether Tokyo intervenes again or shifts toward monetary-policy measures.

For now, Japan retains a huge reserve cushion, but August’s record drawdown has made one fact clear: Tokyo is prepared to spend heavily to prevent another disorderly slide in the yen.

The next test will be whether markets respond to that warning — or challenge Japan’s willingness to intervene again.