The thing with the French military is that they always did what they did REALLY WELL, but they've also always been really slow to adopt and adapt to new methods. So throughout history they tend to be really powerful in the periods right before a significant military advancement, and get wrecked in the periods after one.
Let's ignore the period during the Revolution where the army was trying to fight with pikes, because they thought it was a Patriotic weapon.
Freaking Sans-Culottes. You can't really hold that against France, it's what's to be expected when a nation systematically executes or exiles all its former military leaders. But, hey, they wound up with Napoleon, so I guess it wasn't so bad in the end.
[EDIT]
WW2 is one example, the invention of the English Longbow is a big one, and it took them a pathetically long time to adjust to that one. Their military tradition was too steeped in, well, tradition to allow for the kind of dynamic change needed.
That's great when you need tight execution of a well-established method of attack, but awful when you're facing a new tactic.
Edited, Mar 18th 2014 1:50pm by idiggory