I was just reading a news article about a couple of French men and was wondering what (if any) the difference between that and Frenchmen is?
"Frenchmen" could refer to French people collectively, including French women. For example:
National Stereotypes in Perspective: Americans in France, Frenchmen in America 
It seems that many lower-class Englishmen still harbour a Napoleonic dislike of Frenchmen 
It can, of course, also refer to any smaller group of French persons:
Where would you take a couple of Frenchmen [in L.A. area]? 
"French men" refers just to men from France (all of them or a specific group, depending on context), but not women. Example:
French men are three times more likely than French women to kill themselves 
I think that is the principal difference.
So, in the context of a news article about a couple of people, using "French men" at least makes it clear right away that all of them were men. Using "Frenchmen" would have left that unspecified.
Frenchman means a person, especially a man, who is French by birth or descent (New Oxford American Dictionary); a French man is a man who is from France.
A Frenchman could mean a French ship. That seems an odd usage since ships are usually referred to as 'she', but there you are.
Basically a French man is a Frenchman and vice-versa, so the meaning is the same. Stylistically the word Frenchman seems a little old-fashioned. It's something I would avoid using, perhaps because it is associated with unacceptable terms like Chinaman for Chinese man.