On Thursday, USD/JPY crashed over 500 pips in a matter of hours. Japan’s Ministry of Finance confirmed it had bought yen and sold dollars — its first such operation since 2024.
The yen carry trade is one of the most consequential yet least discussed forces inside the US stock market. For years, investors in the S&P 500 have benefited from it without knowing it existed.
That era is now under serious, structural pressure — and Thursday was not a one-off, but a reminder of yet one more risk that investors need to price.
The invisible engine behind the US bull market
The mechanics are straightforward. Investors borrow yen at Japan’s near-zero interest rates, convert the proceeds into dollars, and park the money in US assets.
The profit comes from the gap between Japanese and American rates, amplified by leverage.
The scale of this trade is estimated anywhere between $4 trillion and $20 trillion, depending on how broadly you measure it.
Every time an investor buys an S&P 500 index fund, some portion of the buying pressure underneath that market has been financed by borrowed yen. US investors have been free riders on Japanese monetary policy for the better part of three decades.
The cheap yen was so reliable, so constant, that it became invisible, just part of the water markets swim in.
The Bank of Japan is now, slowly and deliberately, draining that water.
August 2024 was the warning. Thursday is the reminder.
The clearest preview of what a carry trade unwind looks like came on August 5, 2024.
The BOJ raised rates slightly, the yen strengthened, and traders rushed to unwind positions.
As a result, the Nikkei fell 12% in a single session, its worst day since 1987. The S&P 500 dropped 3%.
The VIX exploded to 65, a level associated with COVID and the 2008 financial crisis, though it recovered quickly because the BOJ blinked almost immediately, walking back its hawkish signals.
That was a partial unwind. What we are dealing with now is a more advanced version of the same setup, except the conditions are worse.
The yen has been weaker for longer, meaning more carry trade has accumulated.
The BOJ is more serious about normalisation this time. And the political environment surrounding the dollar has become genuinely unpredictable.
Thursday’s move, when USD/JPY collapsed from 160.73 to test 155.55 in hours, is the same mechanism, triggered again.
That was the yen’s strongest single-day rally in over three years.
Each intervention is a stress test for carry positions. Each one forces leveraged players to reassess.
The Fed is trapped, and that makes everything worse
Normally, the Federal Reserve would be the stabilising force in a scenario like this. Right now, it is caught between competing pressures that leave it with no clean option.
Inflation is not cooperating. The PCE price index rose 0.7% month-on-month in March, the strongest gain since June 2022.
The Federal Reserve decided to keep rates steady because it cannot credibly cut rates without signalling it has abandoned its inflation mandate.
At the same time, the current US administration has been loudly and publicly pressuring the Fed to ease policy, eroding the perception of institutional independence that underpins the dollar’s safe-haven status.
If the Fed cuts, the dollar weakens, the yen strengthens, and carry trades unwind. If the Fed holds, US growth softens, corporate earnings face pressure, and the stock market weakens anyway.
If the administration successfully undermines Fed credibility, the dollar loses its premium and carry trades unwind for a different reason.
No exit avoids some degree of pressure on the positions that have been quietly financing the bull market.
What US investors are not paying attention to
When carry trades unwind, the selling is fast and indiscriminate. Liquidity evaporates.
The most obvious targets are the largest, and that is most liquid US equities. And this means that big tech gets hit first and hardest. Correlations across asset classes collapse toward one.
Diversification fails exactly at the moment it is needed most. This is what happened in August 2024, briefly, and it recovered because conditions allowed it. The same escape hatch looks narrower today.
There is a specific metric worth watching, which is the USD/JPY itself. A sustained move toward 150 or below would signal genuine stress in carry positions.
It functions as a leading indicator for US equity volatility — more reliable in many ways than the VIX, which tends to react after the fact.
The BOJ currently sits at a 0.75% policy rate against the Fed’s 3.5–3.75%. That gap is still wide, but the direction of travel has changed on both sides. Markets are pricing another BOJ hike by June. Every hike is a slow squeeze. Every squeeze eventually finds a weak point.
Thursday’s intervention did not break the carry trade.
But the regime that made it invisible and safe is cracking, and the US stock market is more exposed to that crack than most investors currently appreciate.
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