This, and not only Napoleon III greatest mistakes but french greatest mistakes in their foreign polivy's history as well.
As historie have proven latter France is not rich or populous enough to fight a unified germany. In revenge, by keeping Germany fragmented France ensured is dominance.
The greatest step toward that unification was the Austro-Prussian war, it allowed Prussia to do 3 things: 1. to kick out of germany the only other great power present in it, 2. To unify Northern Germany as a defacto greater Prussia and 3.to put Bavaria, Baden and Wurtemberg in Prussia sphere of interest.
The mistake is even greater when, like Bismarck himself said ''If whe will have seen the red-pants (french armies) on the Rhine it would have been game over''