For years, the soaring U.S. national debt generated dire warnings that investors largely shrugged off, captivated by low borrowing costs that propelled stock markets to new heights. But the moment of reckoning has arrived. A global bond selloff last week drove yields to their highest levels in two decades, abruptly shifting Wall Street’s focus from the artificial intelligence boom to the mounting debt burden. The message from markets is clear: patience has run out.

The precise threshold at which debt becomes unsustainable has long been debated, especially given the dollar’s enduring status as the world’s primary reserve currency. Yet as RSM Chief Economist Joseph Brusuelas noted, the definition is simple: “When does debt become unsustainable? When the global financial markets say it is. That appears to be happening.” The bond rout extended beyond the United States, with yields in the United Kingdom, France, Germany, and Japan also surging. This synchronized move reflects a broader recognition that governments, since the COVID-19 pandemic, have continued spending as if borrowing costs were still at emergency lows, while deficits have widened as if economies still needed crisis stimulus.

The economic environment, however, has transformed. Central banks have raised interest rates sharply to combat inflation, and the AI boom is channeling hundreds of billions of dollars annually into an economy that has become increasingly resilient to higher rates. Meanwhile, technology giants known as hyperscalers are financing their massive capital expenditures with debt, competing directly with the Treasury Department for bond market dollars. This competition adds upward pressure on yields.

Robin Brooks, a senior fellow at the Brookings Institution, wrote that with public debt already so high in many countries, “it’s only been a matter of time until markets run out of patience. It looks like that’s happening now.” The speed of the selloff prompted the Treasury Department to announce an increase in buybacks of long-dated bonds, a move that briefly lowered yields before they rebounded as investors doubted such financial engineering could stem the tide.

Beyond chronic deficits, several factors converged to trigger the market’s alarm. A proximate cause was the resurgence of oil prices amid the ongoing stalemate between the United States and Iran. With no diplomatic progress in sight, investors expect energy costs to keep inflation elevated, potentially forcing central banks to raise rates further. Federal Reserve Chairman Kevin Warsh has refused to offer forward guidance on how policymakers would respond to future inflation, injecting uncertainty that has added upward pressure on bond yields.

Brusuelas also highlighted what he called an “elephant in the room”: economic populism from both political parties. From the left, it manifests as increased spending; from the right, as tax cuts. Both versions tolerate higher inflation and resist central bank efforts to rein it in. “If such policies go on long enough without a course correction, banking and currency crises tend to follow,” Brusuelas warned. “Global investors understand the end game of such policies.”

Analysts at Capital Economics concurred, noting that bond investors are demanding greater compensation for fiscal, geopolitical, and policy uncertainty — a shift they described as persistent. While the pace of the selloff may not be fully justified by recent events, the market’s concerns are rational given governments show little sign of curbing deficits. This means a higher term premium — the extra return demanded for holding long-term assets — is “fundamentally warranted.” As a result, they expect term premia to remain elevated and bond markets to stay susceptible to renewed volatility in the quarters ahead.

The era of ignoring debt may be over. Wall Street’s main character has changed from AI to debt, and the script is still being written.