The desired pretext was offered on July 3, 1870, by the candidacy of the Prince of Hohenzollern to the throne of Spain. To the French, it seemed that Prussia was updating traditional Habsburg politics. France, having rejected for dynastic reasons the candidacy of a Frenchman, the Duke of Montpensier, was threatened by a German prince. Never had the Emperor, now physically and morally ill, had greater need of the advice of a man of the stature of a statesman and the support of an enlightened public opinion. He had neither.
Liberal Minister Ollivier, wanting to show himself as jealous of national interests as any absolutist minister, wanted to do something big and, pushed by the forces of public opinion which he himself had liberated, he accepted war. as inevitable and prepared for it with a light heart. Faced with the decision of a declaration by the Duke of Gramont, the Minister of Foreign Affairs, before the legislative body, on July 6, 1870, alarmed Europe, gave its support to the efforts of French diplomacy and obtained the cancellation of the candidacy of the Hohenzollerns. This did not enter into the views of the Parisian supporters of the war or of Bismarck, who each wanted to declare war on their own. Gramont's ill-advised request for a guarantee of future good conduct from William I gave Bismarck that opportunity, and the king's refusal was turned into an insult by modifying the telegram. The chamber, despite the desperate efforts of Thiers and Gambetta, voted for war by 246 votes to 10.
France was isolated, as much by the duplicity of Napoleon as by that of Bismarck. The revelation at the diets of Munich and Stuttgart of the written text of Napoleon's claims to the territories of Hesse and Bavaria had since August 22, 1866 alienated southern Germany, and encouraged the southern states to sign a convention with Prussia. . Because of a series of similar blunders, the rest of Europe turned hostile. Russia, which after the Polish insurrection of 1863 sought to draw closer to Prussia, learned with dissatisfaction, by the same indiscretion, how Napoleon kept his promises made at Stuttgart. The hope of revenge for her defeat in 1856 when France was in difficulty decided her to a benevolent neutrality. The revelation of Benedetti's designs in 1867 over Belgium and Luxembourg also ensured an unfriendly neutrality on the part of the United Kingdom.
The Emperor was counting on the alliance of Austria and Italy, with which he had been negotiating since the talks at Salzburg (August 1867). Austria, having suffered at his hands in 1859 and 1866, was not ready and asked for a delay before joining the war while Italy's wavering friendships could only be won by the evacuation of Rome. The "chassepots" of Mentana, the "never" of Rouher, and the hostility of the Catholic Empress for any secret article that could open the doors of the capital to Italy deprived France of its last friend.
The armies of Marshal Leboeuf were no more effective than the alliances of Gramont. The incapacity of high-ranking officers of the French army, the lack of preparation for war of headquarters, the irresponsibility of officers, the absence of a contingency plan and the fact of relying on luck, strategy Previously fruitful for the Emperor, rather than an elaborate strategy, appeared immediately in the insignificant engagement of Saarbrücken. Thus the French army multiplied the defeats and the untapped victories, such as those of Froeschwiller, Borny-Colombey, Mars-la-Tour or Saint-Privat, in particular, to end in the disaster of Metz.
With the surrender of the battle of Sedan, the Empire lost its last support, the army. Paris was left unprotected, with a woman in the Tuileries (Eugénie), a terrified assembly in the Palais-Bourbon, a ministry, that of Palikao, without authority, and the leaders of the opposition fleeing as catastrophe approached. On September 4, 1870, the Republican deputies meeting at the Hôtel de Ville formed a provisional government. The Empire had fallen, the Emperor was a prisoner in Germany and France was entering the era of the Third Republic.