The worst selloff in emerging-market (EM) currencies in five years is beginning to reveal the extent of the fallout from the Federal Reserve's tapering of monetary stimulus, compounded by political and financial instability.

The Turkish lira plunged to a record and South Africa's rand fell yesterday to a level weaker than 11 per dollar for the first time since 2008. Argentine policy makers devalued the peso by reducing support in the foreign-exchange market, allowing the currency to drop the most in 12 years to an unprecedented low.

Investors are losing confidence in some of the biggest developing nations, extending the currency-market rout triggered last year when the Fed first signaled it would scale back stimulus. While Brazil, Russia, India, China, and South Africa were the engines of global growth following the financial crisis in 2008, emerging markets now pose a threat to world financial stability.