Map of the North German Confederation (red), the Southern German States (orange) and Alsace-Lorraine
The causes of the Franco-Prussian War are strongly rooted in the events surrounding the unification of Germany. In the aftermath of the Austro-Prussian War of 1866, Prussia had annexed numerous territories and formed the North German Confederation. This new power destabilized the European balance of power established by the Congress of Vienna in 1815 after the Napoleonic Wars. Napoleon III, then the emperor of France, demanded compensations in Belgium and on the left bank of the Rhine to secure France's strategic position, which the Prussian chancellor, Otto von Bismarck, flatly refused. Prussia then turned its attention towards the south of Germany, where it sought to incorporate the southern German kingdoms, Bavaria, Württemberg, Baden and Hesse-Darmstadt, into a unified Prussia-dominated Germany. France was strongly opposed to any further alliance of German states, which would have significantly strengthened the Prussian military.
In Prussia, some officials considered a war against France both inevitable and necessary to arouse German nationalism in those states that would allow the unification of a great German empire. This aim was epitomized by Prussian Chancellor Otto von Bismarck's later statement: "I did not doubt that a Franco-German war must take place before the construction of a United Germany could be realised." Bismarck also knew that France should be the aggressor in the conflict to bring the southern German states to side with Prussia, hence giving Germans numerical superiority. He was convinced that France would not find any allies in her war against Germany for the simple reason that "France, the victor, would be a danger to everybody – Prussia to nobody," and he added, "That is our strong point." Many Germans also viewed the French as the traditional destabilizer of Europe, and sought to weaken France to prevent further breaches of the peace.
The immediate cause of the war resided in the candidacy of Leopold of Hohenzollern-Sigmaringen, a Prussian prince, to the throne of Spain. France feared encirclement by an alliance between Prussia and Spain. The Hohenzollern prince's candidacy was withdrawn under French diplomatic pressure, but Otto von Bismarck goaded the French into declaring war by releasing an altered summary of the Ems Dispatch, a telegram sent by William I rejecting French demands that Prussia never again support a Hohenzollern candidacy. Bismarck's summary, as mistranslated by the French press Havas, made it sound as if the king had treated the French envoy in a demeaning fashion, which inflamed public opinion in France.
French historians François Roth and Pierre Milza argue that Napoleon III was pressured by a bellicose press and public opinion and thus sought war in response to France's diplomatic failures to obtain any territorial gains following the Austro-Prussian War. Napoleon III believed he would win a conflict with Prussia. Many in his court, such as Empress Eugénie, also wanted a victorious war to resolve growing domestic political problems, restore France as the undisputed leading power in Europe, and ensure the long-term survival of the House of Bonaparte. A national plebiscite held on 8 May 1870, which returned results overwhelmingly in favor of the Emperor's domestic agenda, gave the impression that the regime was politically popular and in a position to confront Prussia. Within days of the plebiscite, France's pacifist Foreign Minister Napoléon, comte Daru was replaced by Agenor, duc de Gramont, a fierce opponent of Prussia who, as French Ambassador to Austria in 1866, had advocated an Austro-French military alliance against Prussia. Napoleon III's worsening health problems made him less and less capable of reining in Empress Eugénie, Gramont and the other members of the war party, known collectively as the "mameluks". For Bismarck, the nomination of Gramont was seen as "a highly bellicose symptom".
The Ems telegram had exactly the effect on French public opinion that Bismarck had intended. "This text produced the effect of a red flag on the Gallic bull", Bismarck later wrote. Gramont, the French foreign minister, declared that he felt "he had just received a slap." The leader of the monarchists in Parliament, Adolphe Thiers, spoke for moderation, arguing that France had won the diplomatic battle and there was no reason for war, but he was drowned out by cries that he was a traitor and a Prussian. Napoleon's new prime minister, Emile Ollivier, declared that France had done all that it could humanly and honorably do to prevent the war, and that he accepted the responsibility "with a light heart". A crowd of 15,000–20,000 people, carrying flags and patriotic banners, marched through the streets of Paris, demanding war. On 19 July 1870 a declaration of war was sent to the Prussian government. The southern German states immediately sided with Prussia.