1. Great performance of US stocks in 2017, particularly for
US citizens, + 19 %, not noticing the erosion of their currency
For a European investor who did not hedge, the FX rate move erased most of the gains

2. Why the European investor did not hedge by selling his/her $ forward ?
Because early January 2017 just after the US presidential election, everyone
was seeing the $ getting even stronger
Quotes from major advisory institutions dated Dec16-Jan17 with DXY near 102 and €/$

3. Well, not everyone actually:
This is how I started 2017 with my daily comment:
My clients, being European for instance,
were made aware of the risk for a weaker $
simply with the observation of a chart pattern.
Consequently they could hedge their US currency exposure before it lost 13 % to the Euro