A currency shakeup is being framed as bad news — the start of a wider global crash that hits weaker economies first while the dollar stays strong.

• The US Treasury reportedly told banks it may step in to buy Japanese yen, after Japan already moved to prop up its own currency.
• The yen had crashed to record lows against the dollar but has bounced back sharply on this intervention.
• The worry is that Japan could be forced to sell its huge pile of US bonds to raise cash, so the US wants to backstop the yen instead.
• This is tied to the unwinding of the "yen carry trade," where investors borrowed cheap yen to buy other assets and are now taking losses.
• The bet is that the euro is the next to fall, weakening the euro zone while money flows into the dollar.

Outlook: Expect more pressure on the euro and other shaky currencies, with the dollar staying strong as global markets wobble.