1.This must be possible only after the Third World War. Without a unified political system there would be no unified currency. Money is a country's credit guarantee, so must have the support of the military, there is no strong military force, you can not even the internationalization of domestic currency.
2.Exchange rate behind the existence of a return on capital, if the return on capital is the same, there is no difference, no longer necessary capital flows, exchange rates disappear, the world datong.