Legend has it that French Fries were introduced to the U.S by soldiers returning from Europe after the Great War. They thought that Belgians were French hence the confusion. That is the story as told by Belgians anyway, but the term French Fries was already in America before then.
I would say that the Belgians probably did invent them as they are not one to boast anyway. Not that it matters. French Fries rolls off the English tongue nicley, and when you are in this part of the world you only need one word: frites.
Frites in the Netherlands are most popular with a big dollop of mayonnaise. It sounds wrong but its not too bad actually.
[Frites and mayo - Maastricht]