I disagree on several points.
The imperial régime was stronger than ever at the eve of the franco-prussian war. The plebiscite of 1870 on the parlementarian evolution of the constitution was a real triumph for Napoleon III. France was going to some kind of british model, but with universal suffrage (since 1848).
What caused the fall of the regime was the disastrous military defeat. Nothing else would have caused it at the time. A wide majority of the country did not trust republican regime because, for them, republican regime meant anarchy and violence (the national trauma of the years 1792-1799 which were a kind of civil war inflicted on the country by a minority of extremists).
The result of the referendum was maybe a landslide, especially compared to the Republican breakout in the previous elections of 1869, but the repartition of votes is more speaking.
The main places where ''no'' votes were a majority were towns, something no surprising given that urban middles are the main source of opposition to the Empire, with Paris, Lyon, Marseille, Bordeaux, Toulouse, among the great towns of the Empire. It is in the countryside that the Imperial Regime found its main source of support, among this traditionnally conservative middle (either loyal to monarchists or to Bonapartists). They are what we could name the ''silent majority'' but were by nature far less active than urban based Republicans; they well tried to oppose replacement of mayors by the Republican dominated National Defense Government during autumn 1870 but were unsuccessfull.
And if we examine the electoral results since 1857 (the election of The Five, first Republicans to enter the parliament since 1851), the Republicans were on the rise. 1869 is the year of the breakout with 3,4 millions to the opposition against only 4,4 millions for the government supported candidates, and only in Paris, it's 234,000 against 77,000 in favour of the opposition.
And you're right to point out to the defeat of Franco-Prussian War as cause for the fall of the Empire. At the time, the Empire was a colossus with feet of clay. The liberal concessions of Napoleon III had weakened his hold on the country (since the free tade agreement of 1860, he lost his traditional base in industrialist and merchant class) and the imperial regime was at a perilous crossroads, it was a ''make-or-break time'' for the Empire.
Had it survived to the war, either with victory or stalemate (officially considered a victory), legitimacy for the Empire would have been highly strengthened. Already, the result of the referendum, a setback after their success in 1869, had scared the Republicans, with Gambetta saying ''the Empire is stronger than ever''.
This legitimacy would have been even more strengthened by the crowning of the Imperial Prince around 1874 (Napoleon III, old, sick and tired planned to abdicate and retire when the Prince would be 18). He was popular and not associated like his father to the overthrow of the Republic. This new youth brought to the Empire would burry deeper and deeper the prospect of Republican restoration and the opposition mantle would be taken up by some other group.
But to come back on the more immediate causes of the Empire fall, the situation in 1870 was still volatile.
The funerals of Victor Noir, journalist mortally shot by a distant scion of the Dynasty (of the Lucien House, pariahs of the Bonaparte dynasty since the days of Napoleon I), trigerred so much uproar that the organizers had a very hard time preventing the funerals from degenerating into uprising (a scenario which to me looks like the funerals of General Lamarque in 1832 and the following uprising immortalized by Victor Hugo in Les Misérables), a scenario as much feared by the government.
And the uprising that led to proclamation of the Third Republic on September 4th was more due to Trochu apathy and deception with Napoleon III, having lost hope with him since he had given up the Plan of Chalons (as I would call it) that Trochu, Prince Napoleon and some members of the Imperial staff had engineered to have the Emperor returning to Paris and put an end to the Regency of Empress Eugénie. Under this plan, Trochu would be appointed Military Governor of the capital, the Emperor would return to Paris, the Regency would be ended, the Army would retreat and use the cover of Paris' defenses to recover. However, at some point, Napoleon III had the idea to ''consult'' his beloved wife who easily convinced him to reject the plan; it is a fact that Eugénie did everything to keep her husband away from the capital and remain Regent. When Trochu, already appointed military governor of the capital, learned of the Emperor's change of mind, he was deeply angered and had this conversation with Prince Napoleon in the afternoon of August 17th:
- All is lost.
- Does it mean that you will help to overthrow our dynasty?
- No, I wouldn't help, but I wouldn't defend it.
Quote from
PLON-PLON, The Red Bonaparte by Michèle Battesti, pp499-500
Actually, the demonstrations which led to the Corps Législatif voting the proclamation of the Republic by a ''reluctant'' Gambetta (he doesn't want to make a coup, reminding the one of 1851), could have easily be dispersed by Paris garrison, but Trochu did nothing to oppose the crowds which converged on the Palais Bourbon and the Gendarmes entrusted with its protection stood aside.
Then, had the Plan of Chalons worked, it would have essentially sidelined Empress Eugénie, and given the weak mind of Napoleon III after the string of defeats, his cousin Prince Napoleon would have likely taken charge of the military matters, having a better reputation within the Army and the Navy than the Emperor himself, Trochu even saying ''there was at Chalons no other Napoleon than him'' (while both the Emperor and his cousin were here).
Post franco-prussian war, the choice of republican regime was only a compromise because monarchiste could not agree on the kind of monarchy.
As far as the war is concerned, Prussia had a very hard time defeating the austrian army (and its german allies) which resisted and fought better than the french army.
So if the french had intervened on the austrian side, the prussians would have been forced to split forces and would certainly have lost the war.
I was not saying that France would have lost; to the contrary, French intervention would have mean Prussian defeat, but even with Prussian forces split, the French Army of 1866 would have had a hard time defeating them in Rhineland (look at what happened in 1870 when the French moved into Germany, not even with their full strength involved), given Prussian far better organization and training.
Though, I don't think that Bismarck would have waited for a defeat in Rhineland. This man was smart enough to recognize a defeat in waiting and ask for a cease fire soon after the French Army would have begun operations. Having already fooled Napoleon III once, he could have tried another time and asked for a peace conference, what Napoleon III would have very willingly agreed to (I don't know why, but he seemed to be a fan of such conferences).