It is very easy actually: the worse the odds are for France, the better they do. Look: in 1805, we were fighting several countries and won, while in the later stages of the war, we failed to slaughter those silly folks from Albion who barely knew what end of a sword was used for business and... I mean, our most friendly neighbours from England.
Similarly, France did well in the Hundred's Year Wars, especially after the Treaty of Troyes which left France in a very weak situation (1420 or so), and the discovery of Cannon resulted in the English tide receding. On the other hand, despite having more well-fed troops with better equipment fighting on our soil, we still managed to lose at Azincourt. Don't ask. Really, don't.
And the story goes on: we successfully fought after the Revolution against all our neighbours, but failed to quench the uprisings in the Low Countries (England didn't do well there either), and... Well, add more examples as appropriate. The bottom line is: if you want to rely on the French, make sure the odds are plain awful.
Of course, it's not as if you can actually win most wars: Napoleon may have lost at the end of the day, but Europe was pretty much the same as in 1800 once the wars were over. The only point seems to be to weaken the enemy... problem is, you will get weaker yourself in the process, preventing you from achieving much, and bringing you on par with the enemy... so another war can be started, to weaken the enemy, and...