For decades, a comfortable global bargain sustained the financial system: The United States ran up massive debts and Japan purchased U.S. Treasuries to cover the bill. This setup granted America cheap borrowing costs and allowed export-driven Japan to accumulate massive dollar reserves while keeping its own interest rates low. Today, that bargain is cracking under the weight of severe currency depreciation. The dramatic decline of the Japanese yen — which recently hit a four-decade low of nearly 160 yen per dollar — is far more than a domestic issue for Tokyo. It is a symptom of deep structural imbalances that could ultimately spark a sovereign debt crisis in the United States.

The root of the yen's decline lies in the widening chasm between the U.S. Federal Reserve's aggressive interest rate hikes and the Bank of Japan’s ultralow rate policy. This disparity triggered a massive yen carry trade, where investors borrowed cheaply in yen to purchase higher-yielding dollar-denominated assets. While this flooded global markets with liquidity, it placed relentless selling pressure on the yen. Compounding the issue, high energy import costs forced Japanese companies to dump yen for dollars, while foreign equity investors heavily hedged their currency exposure.

This currency turmoil directly threatens U.S. Treasury markets because Japan remains the largest foreign holder of American government debt. To defend its crashing currency, Tokyo must buy yen using its foreign reserves, which primarily means selling off its vast holdings of U.S. Treasuries. Reports indicate Japan has begun reducing its foreign securities by tens of billions of dollars. Ordinarily, global markets might absorb these liquidations, but Washington is currently issuing unprecedented volumes of new debt to refinance existing obligations and fund expanding deficits. With total U.S. publicly held debt already surpassing 100 percent of GDP, losing its largest foreign creditor would shift the market dynamics dangerously.

If Japan transforms from a reliable buyer into a persistent seller, Treasury yields will spike, driving up mortgage and corporate borrowing rates across the American economy. An already fragile commercial real estate market could buckle, and higher yields mean the U.S. government's own interest payments will consume an ever-larger share of national revenue, choking out other public spending.

Both nations now find themselves trapped in a risky fiscal gridlock. Japan’s national debt exceeds 250 percent of GDP, meaning raising interest rates to stabilize the yen would make its own debt interest unmanageable. Meanwhile, the Federal Reserve cannot maintain high rates indefinitely without ballooning the U.S. budget deficit, yet cutting rates risks reigniting inflation. This structural fragility serves as a stark warning to the rest of the world, prompting nations across the Global South to look toward alternative payment systems and trade settlements. Ultimately, the weakening yen demonstrates America’s vulnerability in its addiction to debt, paired with an absolute reliance on increasingly cautious foreign creditors. The next financial crisis may not originate on Wall Street — it could begin in Tokyo's currency markets, forcing Washington to finally confront its fiscal excesses.

M. A. Hossain (writetomahossain@gmail.com) is a senior journalist and international affairs analyst, based in Bangladesh.