The immediate pretext for war presented itself when the throne of Spain was offered to a prince of the house of Hohenzollern-Sigmaringen, a branch of the ruling house of Prussia. The offer, at first accepted on Bismarck's advice, was rejected (July 12) after a strong French protest. But the aggressive French foreign minister, the duc de Gramont, insisted on further Prussian assurances, which King William I of Prussia (later Emperor William I) refused. Bismarck, by publishing the famous Ems dispatch, inflamed French feeling, and on July 19, France declared war.

Partly because they believed France the aggressor, the states of S Germany enthusiastically joined the North German Confederation—just as Bismarck had hoped. The military conduct of the war was, for the Germans, in the hands of Helmuth Karl Bernhard von Moltke, a military genius. On the French side, Napoleon III took active command, but it soon devolved on Marshal Bazaine.

On Aug. 4, 1870, the Germans crossed the border into Alsace. They defeated the French at Wissembourg, pushed the French under Marshal MacMahon to Châlons-en-Champagne, and forced a wedge between MacMahon's forces and those of Bazaine, centered on Metz. Bazaine, attempting to join MacMahon, was defeated at Vionville (Aug. 16) and Gravelotte (Aug. 18) and returned to Metz. The Germans began their march on Paris, and on Sept. 1 the attempt of Napoleon III and MacMahon to rescue Bazaine led to disaster at Sedan. The emperor and 100,000 of his men were captured.

When the news of Sedan reached Paris a bloodless revolution occurred. Napoleon was deposed, and a provisional government of national defense was formed under General Trochu, Léon Gambetta, and Jules Favre. Paris was surrounded by the Germans on Sept. 19, and a grueling siege began. Gambetta escaped from Paris in a balloon to organize resistance in the provinces. Faidherbe made a gallant stand on the Loire, Chanzy in the north, and Bourbaki in the east, but the surrender (Oct. 27) of Bazaine, with a garrison of 180,000 men, made such resistance useless. Paris, however, held out until Jan. 28, 1871, suffering several months of famine. Though Bismarck and Adolphe Thiers signed an armistice on the same day, the fortress of Belfort resisted until Feb. 16.

In the war's aftermath, Thiers was named chief of the executive power in France, and provision was made for the election of a French national assembly, which met at Bordeaux. The assembly accepted (Mar. 1) the preliminary peace agreement, which was formalized in the Treaty of Frankfurt (ratified May 21, 1871). France agreed to pay an indemnity of $1 billion within three years—an indemnity fully paid before the term expired. Alsace, except the Territory of Belfort, and a large part of Lorraine were ceded to Germany, which on Jan. 18, 1871, in the Hall of Mirrors at Versailles had been proclaimed an empire under William I.

Paris refused to disarm and to submit to the Thiers regime, and the Commune of Paris was formed. The French troops loyal to Thiers began the second siege of Paris (Apr.-May, 1871). After the cruel suppression of the commune, peace returned to France.

Besides establishing the Third French Republic and the German Empire, the Franco-Prussian War had other far-reaching effects. Desire for revenge guided French policy for the following half-century. Prussian militarism had triumphed and laid the groundwork for German imperialistic ventures. The Papal States, no longer protected by Napoleon III, were annexed by Italy, which thus completed its unification. These and other effects were links in the chain of causes that set off World War I.