The Bank of Japan has spent decades as the world’s most cautious central bank. On Monday, it picked up two weapons and pointed them at the market.

One: the strongest yen intervention warning in years, delivered by top currency diplomat Atsushi Mimura, who deployed the word “decisive” for the first time since taking office. Two: a board member explicitly floating a larger-than-expected rate hike if the Middle East conflict keeps dragging oil prices higher.

The proximate cause is the yen’s slide past 160 to the dollar — its weakest since July 2024, when Japan last spent billions propping up the currency. The underlying cause runs deeper: the Iran war has effectively shut the Strait of Hormuz, choking off roughly a fifth of global oil and gas flows, driving crude prices higher, and sending investors scrambling into the safe-haven dollar.

What makes this moment different is that the BOJ is no longer pretending geopolitics is someone else’s problem.

Stagflation Shadows

At the BOJ’s March policy meeting — where the board kept rates steady at 0.75%, already a 30-year high — the debate behind closed doors was considerably sharper than the hold suggested.

One board member warned that broadening cost pressures from high oil prices could tip Japan into the kind of stagflation it experienced in the 1970s: a slumping economy paired with rising prices simultaneously. Another argued that the BOJ “may need to accelerate the pace of rate hikes and shift toward neutral or restrictive” monetary policy if the Middle East conflict persists, according to the meeting summary released Monday.

A third voice cautioned that the bank risked falling behind the curve, since “second-round effects and rise in underlying inflation stemming from overseas developments are more likely to emerge.”

The thread connecting these arguments: an inflation problem imported from a war zone, and a growing sense that the window for gradualism is closing. As one member put it: “With the policy interest rate still far away from the neutral interest rate, falling behind the curve would compel the bank to pursue rapid and significant monetary tightening, which would cause a major shock to Japan’s economy.”

Governor Kazuo Ueda reinforced the message in Parliament on Monday. Currency moves “are obviously among factors that hugely affect economic and price developments,” he said, keeping alive the chance of a rate hike as soon as April. He added that the BOJ must raise rates at an “appropriate pace” to prevent bond yields from overshooting.

The Intervention Red Line

On the currency side, Mimura delivered what traders read as the most explicit intervention signal to date. “We are hearing that speculative moves are increasing in the currency market, in addition to the crude futures market,” he told reporters. “If this situation continues, it may be time to take decisive measures.”

That word — “decisive” — is the one currency traders had been waiting months to hear from Mimura specifically. Finance Minister Satsuki Katayama had used it in recent weeks, but Mimura, who actually oversees currency policy, had not deployed it publicly since taking office in July 2024. He chose Monday to change that, and the market noticed.

The last time Japan intervened, in mid-2024, the yen was trading near similar levels. The conditions for a repeat are now firmly in place.

Why the World Should Care

This is not just a Japan story. The yen has long been the funding currency of choice for carry trades — the strategy of borrowing cheaply in yen to invest in higher-yielding assets elsewhere. A weaker yen makes those trades more profitable. A sudden yen strengthening — whether from direct intervention or a surprise rate hike — can unwind them violently.

The tremors are already visible. Japan’s 10-year government bond yield hit a 27-year high on Monday, according to Channel News Asia. The Nikkei stock average fell as stagflation fears took hold. If the BOJ follows through on its rhetoric, the ripples will not stay confined to Tokyo.

This is a central bank that spent the better part of two decades in monetary stimulus mode, clinging to negative rates while the rest of the developed world tightened. It ended that era in 2024. Now it is telling markets explicitly that a war in the Middle East could dictate Japanese interest rate policy. The BOJ has, in effect, admitted that geopolitical conflict is now a monetary policy variable — and for an institution built on caution, that admission is itself the signal.

Sources