Every year, the U.S. Treasury Department reports to Congress on whether major trading partners are manipulating their currencies. In October 2017, the Treasury declined to name any major trading partners as currency manipulators. To be a currency manipulator, a country must have a large trade surplus with the United States a large overall trade surplus, and intervene in the foreign currency. 

Explain why this definition requires foreign exchange intervention in order to be classified as a currency manipulator.