For the record, my initial answer was much more reasonable. Our exchange rate is just a price — the price of the dollar in terms of other currencies. It is not controlled by anyone. And a high price for the dollar, which is what we mean by a strong dollar, is not always desirable.

Some countries, like China, essentially fix the price of their currency. But since the early 1970s, the United States has let the dollar’s value move in response to changes in the supply and demand of dollars in the foreign exchange market. The Treasury no more determines the price of the dollar than the Department of Energy determines the price of gasoline. Both departments have a small reserve that they can use to combat market instability, but neither has the resources or the mandate to hold the relevant price away from its market equilibrium value for very long.

In practice, all that “the exchange rate is the purview of the Treasury” means is that no official other the Treasury secretary is supposed to talk about it (and even he isn’t supposed to say very much). That strikes me as a shame. Perhaps if government officials could talk about the exchange rate forthrightly, there would be more understanding of the issues and more rational policy discussions.

Such discussions would start with some basic economics. The desire to trade with other countries or invest in them is what gives rise to the market for foreign exchange. You need euros to travel in Spain or to buy a German government bond, so you need a way to exchange currencies.

The supply of dollars to the foreign exchange market comes from Americans who want to buy goods, services or assets from abroad. The demand for dollars comes from foreigners who want to buy from the United States.