information:extraction, structural:reformatting
Franco-Prussian War 1870–71

Prussia's victory over Austria increased the already existing tensions with France. The Emperor of France, Napoleon III, had tried to gain territory for France (in Belgium and on the left bank of the Rhine) as a compensation for not joining the war against Prussia and was disappointed by the surprisingly quick outcome of the war. Accordingly, opposition politician Adolphe Thiers claimed that it was France, not Austria, who had really been defeated at Königgrätz. Bismarck, at the same time, did not avoid war with France, though he feared the French for a number of reasons. First, he feared that Austria, hungry for revenge, would ally with the French. Similarly, he feared that the Russian army would assist France to maintain a balance of power. Still, however, Bismarck believed that if the German states perceived France as the aggressor, they would then unite behind the King of Prussia. To achieve this, he kept Napoleon III involved in various intrigues, whereby France might gain territory from Luxembourg or Belgium. France never achieved any such gain, but it was made to look greedy and untrustworthy.

A suitable pretext for war arose in 1870, when the German Prince Leopold of Hohenzollern-Sigmaringen was offered the Spanish throne, vacant since a revolution in 1868. France pressured Leopold into withdrawing his candidacy. Not content with this, Paris demanded that Wilhelm, as head of the House of Hohenzollern, assure that no Hohenzollern would ever seek the Spanish crown again. To provoke France into declaring war with Prussia, Bismarck published the Ems Dispatch, a carefully edited version of a conversation between King Wilhelm and the French ambassador to Prussia, Count Benedetti. This conversation had been edited so that each nation felt that its ambassador had been slighted and ridiculed, thus inflaming popular sentiment on both sides in favor of war. Langer, however, argues that this episode played a minor role in causing the war.

Bismarck wrote in his Memoirs that he "had no doubt that a Franco-German war must take place before the construction of a united Germany could be realised". Yet he felt confident that the French Army was not prepared to give battle to Germany's numerically larger forces: "If the French fight us alone they are lost". He was also convinced that the French would not be able to find allies since "France, the victor, would be a danger to everybody – Prussia to nobody". He added, "That is our strong point".

France mobilized and declared war on 19 July. The German states saw France as the aggressor, and—swept up by nationalism and patriotic zeal—they rallied to Prussia's side and provided troops. Both of Bismarck's sons served as officers in the Prussian cavalry. The war was a great success for Prussia as the German army, controlled by Chief of Staff Moltke, won victory after victory. The major battles were all fought in one month (7 August to 1 September), and both French armies were captured at Sedan and Metz, the latter after a siege of some weeks. Napoleon III was taken prisoner at Sedan and kept in Germany for a time in case Bismarck had need of him to head the French regime; he later died in exile in England in 1873. The remainder of the war featured a Siege of Paris, the city was "ineffectually bombarded"; the new French republican regime then tried, without success, to relieve Paris with various hastily assembled armies and increasingly bitter partisan warfare.

Bismarck quoted the first verse lyrics of " La Marseillaise ", amongst others, when being recorded on an Edison phonograph in 1889, the only known recording of his voice. A biographer stated that he did so, 19 years after the war, to mock the French.