The causes of the Franco-Prussian War are deeply rooted in the events surrounding German unification. In the aftermath of the Austro-Prussian War (1866), Prussia had annexed numerous ethnically German territories and formed the North German Confederation with other German territories. Prussia then turned its attention towards the south of Germany, where it sought to expand its influence.
France was strongly opposed to the annexation of the Southern German States (Bavaria, Wurttemberg, Baden and Hesse-Darmstadt) by the North German Confederation, which would have created too powerful a country next to its border. In Prussia, a war against France was deemed necessary to arouse German nationalism in those States in order to allow the unification of most German states (excluding the ethnically German lands of the Austrian Empire) into a great German empire. This aim was epitomized by Prussian Chancellor Otto von Bismarck's quote: "I knew that a Franco-Prussian War must take place before a united Germany was formed."Otto von Bismarck (A.J. Butler, trans.), Bismarck: The Man and the Statesman, vol. 2, page 58. Originally published in 1898; reprinted in 2007 by Cosimo Classics of New York, New York. 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.
The immediate cause of the war resided in the candidacy of 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 The Hohenzollern prince's candidacy was withdrawn under French diplomatic pressure, but Otto von Bismarck goaded the French into declaring war by altering a dispatch sent by William I. Releasing the Ems Dispatch to the public, Bismarck made it sound as if the king had treated the French envoy in a demeaning fashion. Six days later, France declared war on Prussia and the Southern German States immediately sided with Prussia.
French Emperor Napoleon III and Prime Minister Émile Ollivier's eagerness to relieve France from internal political convulsions also contributed to France's declaration of war on Prussia.
In a book published in connection with a symposium held on the 150th anniversary of the Franco-Prussian War in 2020, Nicolas Bourguinat and Gilles Vogt, co-organisers of the symposium, point out that most French historians now cast into doubt the three-point official version regarding the causes of the war that has dominated French historiography for a long time: 1) that Bismark needed a war to bring Southern Germany behind Prussia in order to complete German unification; 2) that Bismarck lured France into a trap in connection with a candidacy to the Spanish throne; 3) that the French government, pressured by public opinion and convinced of its military superiority, fell head-on into the trap."Depuis la publication des documents diplomatiques français au début du XXe siècle jusqu’aux études conduites à l'époque du centenaire de la guerre, a longtemps eu cours une vulgate, plus ou moins bien assimilée, déclinable en trois temps: 1) Bismark avait besoin d'un conflit armé pour entraîner l'Allemagne du Sud derrière la Prusse et ses satellites afin de parachever la construction politique de l'Allemagne unifiée; 2) Bismark a attiré la France dans un piège à partir d'un différent portant sur la succession au trône d'Espagne; 3) La France est tombée les yeux fermés dans ce piège, entraînée sans doute par une opinion publique en effervescence, et convaincue de sa supériorité militaire." Nicolas Bourguinat, Gilles Vogt, La guerre franco-allemande de 1870 - Une histoire globale, Flammarion, Paris, 2020, p. 15-16. The authors stress that with the benefit of hindsight it is now difficult to support the thesis that the Hohenzollern candidacy had been a ploy destined to draw France into an armed conflict. Nicolas Bourguinat, Gilles, Vogt, p. 34. Regardless of what might have been Bismarck’s ulterior motives with respect to the Hohenzollern candidacy, Paris, intent on war, had decided early on to use that candidacy as a casus belli. Nicolas Bourguinat, Gilles, Vogt, p. 46.
European wars and the balance of power: 1865–1866 In October 1865, Napoleon III, ruler of France, met with Prussian Prime Minister Otto von Bismarck in Biarritz, France. It was there that the two men struck a deal— France would not get involved in any future actions between Prussia and Austria or ally herself with Austria if Prussia somehow won the war and did not allow Italy to claim Venetia. When Austria and Prussia met in May 1866, Bismarck honored the agreement made in Biarritz the previous year and refused to allow Austria to have Venetia. Austria then attempted to guarantee Italy Venetia if they remained neutral, but the two nations were unable to agree on a suitable arrangement as an alliance formed earlier in the year bound Italy to Prussia. Napoleon III then committed a serious blunder by agreeing with Austria in a secret treaty to remain neutral in an Austrian-Prussian conflict in exchange for France acquiring Venetia plus the establishment of a neutral (i.e., French-leaning) state west of the Rhine; this violated the agreement Napoleon had made with Bismarck.
After Prussia emerged victorious over the Austrian army at the Battle of Königgrätz (also known as Sadowa or Sadová) in the Austro-Prussian War of 1866, negotiations were being held between Austria and Prussia in July and August of that year. It was during that period that Napoleon III first discovered that a bladder stone was causing him great pains, created from gonorrheal infection. His condition was so bad during those negotiations that he was forced to retire to Vichy to recuperate, removing himself from Paris. Although the emperor favored neutrality as to not upset events, certain members of his circle thought it was an unwise move, considering the opportunity to prevent Prussia from becoming too strong. One of these men, foreign minister Édouard Drouyn de Lhuys, convinced the emperor to plant 80,000 men on the eastern border to convince Wilhelm I to maintain the balance of power in Europe. Despite this important victory, de Lhuys was subverted by several other ministers, and Napoleon III changed his mind, reverting to a position of neutrality. This change of heart would end up causing de Lhuys to ultimately lose his position.Jerrold(1883). pp. 327–330 Napoleon III's wife Empress Eugénie, who took an active part throughout his rule, referred to this time much later as "the critical date, the Empire's fatal date; it was during these months of July and August that our fate was sealed! Of all that period, there is not a single fact, not a single detail that has not remained in my mind."Bresler(1999). p. 340
Franz Joseph of Austria accepted Bismarck's terms under the Peace of Prague. Using this to his advantage, Bismarck declared the German Confederation of 1815 null and void, and created a new network of states under Prussian control. Frankfurt-am-Main, Hannover, Hesse-Kassel (or Hesse-Cassel), Holstein, Nassau, and Schleswig were annexed outright while Hesse-Darmstadt, Mecklenburg, Saxony, the Thuringian duchies, as well as the cities of Bremen, Hamburg, and Lübeck were combined into a new North German Confederation that governed nominally and was actually controlled by Prussia herself.
Bismarck was approached soon after the end of the war by Napoleon III's ambassador to Prussia, Vincent Benedetti. Benedetti brought with him a secret proposal by Napoleon III that France would approve of Bismarck's acquisition of the northern German states if Prussia remained neutral while France annexed Belgium and Luxembourg. France had earlier guaranteed the independence of Belgium in the Treaty of London in 1839 as an "independent and perpetually neutral state", making the proposal a tacit agreement to break their promise. Bismarck was very surprised since he had already gained a powerful position in Europe by the armistice, and called Napoleon III's request among others later "like 'an innkeeper's bill' or a waiter asking for 'a tip'." He asked Benedetti to provide the proposal in writing, and the ambassador obliged his request. This document was to be important to Bismarck later on, to great effect.Bresler(1999). pp. 338–339
The true views of Napoleon III on the subject of the balance of power in Europe can be found in a state circular handed to every diplomatic representative for France. In this paper dated September 1, 1866, the emperor saw the future of Europe after the Peace of Prague in this manner:
"Policy should rise superior to the narrow and mean prejudices of a former age. The Emperor does not believe that the greatness of a country depends upon the weakness of the nations which surround it, and he sees a true equilibrium only in the satisfied aspirations of the nations of Europe. In this, he is faithful to old convictions and to the traditions of his race. Napoleon I foresaw the changes which are now taking place on the continent of Europe. He had sown the seeds of new nationalities: in the Peninsula, when he created the Kingdom of Italy; and in Germany, when he abolished two hundred and fifty three separate states."Jerrold(1882) p. 332
Domestic agenda in France and Prussia
French prestige and politics
France's position in Europe was now in danger of being overshadowed by the emergence of a powerful Prussia, and France looked increasingly flat-footed following Bismarck's successes. In addition, French ruler Napoleon III was on increasingly shaky ground in domestic politics. Having successfully overthrown the Second Republic and established the Bonapartist Second Empire, Napoleon III was confronted with ever more virulent demands for democratic reform from leading republicans such as Jules Favre, along with constant rumours of impending revolution. In addition, French aspirations in Mexico had suffered a final defeat with the execution of the Austrian-born, French puppet Emperor Maximilian I of Mexico in 1867.Bresler(1999), p. 345
The French imperial government now looked to a diplomatic success to stifle demands for a return to either a republic or a Bourbon monarchy. A war with Prussia and resulting territorial gains in the Rhineland and later Luxembourg and Belgium seemed the best hope to unite the French nation behind the Bonapartist dynasty. With the resulting prestige from a successful war, Napoleon III could then safely suppress any lingering republican or revolutionary sentiment behind reactionary nationalism and return France to the center of European politics.Wawro(2003), p. 30
Bismarck and German nationalism Prussia in turn was also beset with problems. While revolutionary fervour was far more muted than in France, Prussia had in 1866 acquired millions of new citizens as a result of the Austro-Prussian War,Wawro(2003), p. 17 which was also a civil war among German states. The remaining German kingdoms and principalities maintained a steadfastly parochial attitude towards Prussia and German unification. The German princes insisted upon their independence and balked at any attempt to create a federal state that would be dominated by Berlin. Their suspicions were heightened by Prussia's quick victory and subsequent annexations.Taylor(1988), pp. 84-85. Before the war, only some Germans, inspired by the recent unification of Italy, accepted and supported what the princes began to realise, that Germany must unite in order to preserve the fruit of an eventual victory.Taylor(1988), pp. 70-71.