Panzerde, this got me thinking ab out Napoleon III.
I can't decide. Would you think his greatest mistake was letting Prussia win a total military victory in 1866?
Or, by ignoring centuries of French military policy, letting Piedmont and Cavour unify Italy?
Or His fixation on weakening Austria, to have been his single greatest mistake?
As pointless as the attempted imperial ambitions in Mexico were, that was ultimately a sideshow.
I can't completely fault him about the Austria thing. Messing with the Hapsburgs, of all stripes, is a recurrent theme among French regimes for hundreds or years. It wasn't bright, but it's almost something I'm not sure he could have avoided doing. Having said that I believe his stance vis a vis Austria leads pretty directly to the two other mistakes you mention.
Allowing the Prussian victory in 1866 probably has the most obvious and immediate consequences. Without the 1866 victory over Austria you don't get the 1870 victory that ultimately unseats Napoleon III. Unified Italy seems to have less immediate negative connotations, but as you observe, there were solid reasons not allowing this was French policy for centuries.
As I say, though, I think this can be attributed to his policy toward Austria. He could have backed off the ancient dynastic struggle, maintained a French hand in Italy and probably, with Austrian help, prevented or delayed Prussian hegemony in Germany. His fixation on Austria led him to miss the real threat to the Second Empire and to fritter away Imperial resources in Italy. So, even though it was somewhat in keeping with his predecessors to oppose Austria I have to see that as the core of his eventual failure. Even the Mexican thing traces back to it.
What's your spin Besilarius? I'll be the first to admit that I'm less well read on the Second Empire than I am a couple of hundred years earlier, so the above might be pretty off base!