What factors lead to the disappearance of military helmets from European armies during the 17th through 19th centuries, and then to their return to use in the early 20th century? To what extent were these reasons technical as compared to stylistic or fashion, and were other factors involved as well?
For example, during the Napoleonic period shakos, bearskins and similar headgear were the most common, while 200 to 150 years earlier (the Thirty Years War and Eighty Years War) helmets were commonly worn by pike-men and musketeers.
EDIT: In addition, does anyone have rough dates as to when the helmet re-appeared in European armies?