The Franco-Prussian War or Franco-German War, often referred to in France as the 1870 War (19 July 1870 – 10 May 1871), was a conflict between the Second French Empire and the Kingdom of Prussia. Prussia was aided by the North German Confederation, of which it was a member, and the South German states of Baden, Württemberg and Bavaria. The complete Prussian and German victory brought about the final unification of Germany under King Wilhelm I of Prussia. It also marked the downfall of Napoleon III and the end of the Second French Empire, which was replaced by the French Third Republic. As part of the settlement, the territory of Alsace and part of Lorraine was taken by Prussia to become a part of Germany, and it remained so until the end of World War I when it was returned to France in the Treaty of Versailles.

The conflict was a culmination of years of tension between the two nations, which finally came to a head over the issue of a Hohenzollern candidate for the vacant Spanish throne, following the deposition of Isabella II in 1868. The public release of the Ems Dispatch, which played up alleged insults between the Prussian king and the French ambassador, inflamed public opinion on both sides. France mobilized, and on 19 July 1870 declared war on Prussia only, but the other German states quickly joined on Prussia's side.