Why the Japanese bond drama matters for the global economy

The financial headlines overnight were full of speculation about currency interventions by the Japanese and U.S. governments. That's really a small piece of a bigger story — one with serious consequences for the global economy.

The big picture: Japan has played an outsize role in the global financial system over the last two decades, with its massive pile of low-interest debt and major institutional investors looking to deploy capital for higher returns.

• Those underpinnings are now coming into question. Should they unravel more fully, it could throttle capital inflows to the rest of the world and therefore lower asset values and lift long-term rates.
• It raises the prospect of a world where bond investors are more jittery about buying the debt of countries with large ongoing fiscal deficits, like a certain global superpower celebrating its 250th birthday this year.

Driving the news: The yen soared 1.2% against the U.S. dollar overnight, following a steep sell-off last week, as traders saw hints that a government intervention to prop up the currency could be imminent.

• "We will take all necessary measures to address speculative and highly abnormal movements," Japanese finance ministry official Atsushi Mimura said Sunday, per Bloomberg.
• It comes after a week of wild gyrations in both the currency and longer-term Japanese bonds, which sold off sharply last Monday and Tuesday, pushing borrowing rates sharply higher — with spillover effects to U.S. assets.

State of play: Japan has spent the last three decades trying to jolt its economy out of a deflationary trap that has been accompanied by persistently low interest rates.

• Those low rates have allowed the Japanese government to run up an enormous public debt of more than 200% of GDP.
• It has contributed to capital flows abroad. Japanese insurers and pension funds have plowed money into higher-returning assets overseas. Hedge funds have maintained carry trades, borrowing in yen and investing in higher-yielding assets elsewhere.
• The Japanese government's war on deflation increasingly looks to have been won — but nobody said the journey from the ultra-low rates regime to something more normal would be seamless.

Zoom out: The recent volatility began with Prime Minister Sanae Takaichi proposing tax cuts in the run-up to Feb. 8 elections. That, in turn, fueled worries about widening deficits and their impact on Japan's fiscal sustainability in light of the massive existing debt overhang.

• It is a turnabout from decades when the government could seemingly enjoy a policy-free lunch, running loose monetary policy and wide deficits with little pain in the form of inflation or higher rates.
• Key longer-term rates spiked to multi-decade highs. They have since receded, but the 40-year Japanese government bond is now yielding 3.91%, up from 2.65% a year ago.
• That in turn triggered a large (though short-lived) sell-off of U.S. assets, reflecting the heightened risk of a pullback of Treasury buying by Japanese institutions and the yen carry trade — and broader wariness of countries with high debt.

What they're saying: "Japan has been the world's financial shock absorber for a generation, and that role has abruptly ended," Nigel Green, CEO of deVere Group, says in a note.

• "The repricing of Japanese debt is a systemic event, not a local story, and investors need to treat it as such," he adds.

The bottom line: "The bond market is signaling a credibility test for fiscal policy," Green writes.