On most days, Kazakhstan finds itself in the backwaters of financial markets. Yet, it’s this central Asian nation that has delivered the latest shock to global currency trading. Thursday’s 23 percent plunge in the tenge after Kazakhstan abandoned control of its exchange rate revealed a sense of urgency among policy makers: they had tried a managed depreciation just a day earlier. The escalation signaled to investors that it has become too costly for developing nations to defend their currencies. Vietnam also devalued the dong, while freely traded currencies such the South African rand and Turkey’s lira extended losses. The trigger for the wave of depreciations was China’s decision to weaken the yuan on Aug. 11, leaving countries competing with the world’s second-largest economy in export markets and those selling goods to it at a disadvantage. That added to the woes of emerging markets already reeling from a looming increase […]