Trying to assess whether a global currency war is under way is not as easy as it sounds. For example, changes in the yen-dollar cross (exchange rate) might tell you if the Japanese currency is weakening, but the number is less useful if both the Bank of Japan and the Fed are devaluing their money. The spending power of the U.S. dollar and the yen may both be declining at an equal rate, in which case the exchange rate will not change.