Not really what you asked for, but what I would say is that this specific incident isn't like some huge deal or something. It's just the USA doing something that helps Japan stabilize its currency, and helps the USA avoid a spike in people selling US treasuries (which would raise US borrowing costs).

It's unusual, but not earth shattering or crazy.

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The wider picture looks rather worrysome though. Japan has spent decades building up a nest egg of US treasuries as a way to try and fight of deflation. Now, they have inflation and currency depreciation, so the extremely natural thing to do is for Japan to sell their accumulated assets to defend their currency and dampen inflation.

The USA on the other hand has been going around with a fork and sticking it in electrical sockets, and has earned a reputation for being extremely erratic and unfocused on stability. The USA also has zero plan or intention to get its debt burden under control.

This makes investors who hold US treasuries nervous. They see increasing geopolitical instability, increasing political disfunction in the USA, and the early stages of a USA debt crisis that could end in debt defaults (Bessent has already actually hinted at this, when he suggested unilaterally converting some already sold bonds to '100 year bonds').

This situation has caused US borrowing costs to go up, and japan switching from a net treasury buyer to a net treasury seller would make it harder for the USA to sell more bonds without giving even higher interest rates, which just makes the current debt troubles worse.

Here’s the likely rationale from one of the FT comments:

“It looks like the Japanese economy is on the BoJ [Bank of Japan] ventilators. I mean, the BoJ is the largest single holder of Japanese equities, government bonds (JGBs) and currency (JPY). It’s likely that the BoJ is printing more yen to finance Japan Inc, which in turn is probably the driving force behind inflation.

[…]

BoJ is the largest foreign bank holding USTs [US treasuries], around $1.14tn, it’s likely that they would have had to sell some treasuries to finance JPY purchases. My view is that, this scenario is not ideal for the US Treasury – particularly right now with the UST yield curve steepening – hence they had to “return the favour” by selling EURJPY”

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tl;dr in my layman interpretation: US helps Japan by selling (shorting) EU in a debt-exchange triangle. The US didn’t have much choice, as Japan would have sold USD, which they hold plenty of, to finance their spending spree. They just have to hope their bet on JPY vs EUR pays off in the long-term.

Why would the sky be falling? Getting the world's largest economy to prop up your own economy is kind of the point to being an ally of the biggest economy in the world; the Japanese and American governments being in bed together and planning the Japanese economy is not just normal, but is one of the bigger conspiracy theories persisting from the 1980s when US intervention is blamed as the reason Japan's economy stagnated in the 90s. So I don't understand why this would be a sky is falling moment.

FX interventions are nothing new and 3% isn't nothing but it's not that much. The sun will rise in the morning, nbd.