The U.S. is due to decide this week whether to designate China a currency manipulator. While it’s not expected to apply the label on this go around, the yuan has fallen more than 9% against the dollar since it last considered the move in April.

Tensions between the two nations have been building since then. It is tempting to think that China must be actively pushing its currency down in an attempt to offset the effect of the 10% tariffs being imposed on its exports to the U.S. by President Donald Trump. It is also tempting...