information:classification, structural:scholar_edition
Causes

The causes of the Franco-Prussian War are rooted in the events surrounding the lead up to the unification of the German states under Otto von Bismarck. France had gained the status of being the dominant power of continental Europe as a result of the Franco-Austrian War of 1859. During the Austro-Prussian War of 1866, the Empress Eugénie, Foreign Minister Drouyn de Lhuys and War Minister Jacques Louis Randon were concerned that the power of Prussia might overtake that of France. They unsuccessfully urged Napoleon to mass troops at France's eastern borders while the bulk of the Prussian armies were still engaged in Bohemia as a warning that no territorial changes could be effected in Germany without consulting France.

As a result of Prussia's annexation of several German states which had sided with Austria during the war and the formation of the North German Confederation under Prussia's aegis, French public opinion stiffened and now demanded more firmness as well as territorial compensations. As a result, Napoleon demanded from Prussia a return to the French borders of 1814, with the annexation of Luxembourg, most of Saarland, and the Bavarian Palatinate. Bismarck flatly refused what he disdainfully termed France's politique des pourboires ("tipping policy"). He then communicated Napoleon's written territorial demands to Bavaria and the other southern German states of Württemberg, Baden and Hesse-Darmstadt, which hastened the conclusion of defensive military alliances with these states. France had been strongly opposed to any further alliance of German states, which would have threatened French continental dominance.

The only result of French policy was the consent of Prussia to nominal independence for Saxony, Bavaria, Wurttemberg, Baden, and Hessia-Darmstadt; this was a small victory, and one without appeal to a French public which wanted territory and a French army which wanted revenge. The situation did not suit either France, which unexpectedly found itself next to the militarily powerful Prussian-led North German Confederation, or Prussia, whose foremost objective was to complete the process of uniting the German states under its control. Thus, war between the two powers since 1866 was only a matter of time.

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 four 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 was the candidacy of Leopold of Hohenzollern-Sigmaringen to the throne of Spain. France feared an encirclement resulting from 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 of 13 July 1870 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. French mobilization was ordered early on 15 July. Upon receiving news of the French mobilization, the North German Confederation mobilized on the night of 15–16 July, while Bavaria and Baden did likewise on 16 July and Württemberg on 17 July. On 19 July 1870, the French sent a declaration of war to the Prussian government. The southern German states immediately sided with Prussia.

Napoleonic France had no documented alliance with other powers and entered the war virtually without allies. The calculation was for a victorious offensive, which, as the French Foreign Minister Gramont stated, was "the only way for France to lure the wary Austrians, Italians and Danes into the French alliance". The involvement of Russia on the side of France was not considered by her at all, since Russia made the lifting of restrictions on its naval construction on the Black Sea imposed on Russia by the Treaty of Paris following the Crimean War a precondition for the union. But Imperial France was not ready to do this. "Bonaparte did not dare to encroach on the Paris Treaty: the worse things turned out in the present, the more precious the heritage of the past became".