The US and Japan teamed up to prop up the collapsing yen, and it's a sign of how fragile the whole system has become — bad news for currency traders and anyone counting on stable bond markets.

• The US helped rescue Japan's crashing yen, which snapped back hard and wiped out traders betting against it.
• To do it, the US sold euros instead of dollars, avoiding a panic signal that the dollar itself is in trouble.
• The real fear is the "carry trade" — traders borrow cheap yen to buy US bonds, and an unwind could dump trillions in US debt and spike interest rates.
• A quiet repo facility lets Japan swap its US bonds for cash without dumping them on the market, containing the damage for now.
• The deeper motive is keeping Japan buying US chips and bonds — Japan plans to buy tens of thousands of Nvidia chips and pour billions into manufacturing.

Outlook: The intervention buys time, but the yen is still set to weaken and Japan will likely be pushed to buy more US debt to repay the favor.