Inhabitants of what is today France have been renowned for an exceptional martial culture from at least the time of the Gauls (before Roman conquest), to the Frankish Empire, to Napoleon, and France was a major power throughout the Imperial Age until the Great War (and still is, one of the worlds strongest military- and nuclear powers).
Germany rolled over France in the Franco-Prussian War and in WWII, and America (which owes its existance in no small part to France), and much of Europe seeing a recent display of the shift in mainland power balance, have had a social trend to categorize them as cowards ever since the Blitzkrieg, where in truth probably no country in the world could hope to stand against the top-modern tactics, equipment and personnel of the German army anno 1940, if being so unfortunate as to share borders with the Third Reich.
Francophobia in the US stems from the 2003 French refusal to join the illegal war against Iraq ('freedom fries'), and has also been inherited from the English.