The Franco-Prussian War or Franco-German War (19 July 1870-10 May 1871) was a conflict between France and Prussia, which was backed by the North German Confederation and the south German states of Baden, Württemberg and Bavaria. The thorough German victory brought about the final unification of the German Empire under King William I of Prussia. It also marked the downfall of Napoleon III and the end of the French Second Empire, which was replaced by the Third Republic. As part of the settlement, the territory of Alsace-Lorraine was taken by Germany, which would retain it until World War I.
The conflict was the culmination of years of tension between the two powers, 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 declared war on Prussia only, but the other German states quickly joined on Prussia's side.
The superiority of the German forces was soon evident, due in part to efficient use of railroads and innovative Krupp artillery. A series of swift German victories in eastern France culminated in the Battle of Sedan, at which Napoleon III was captured with his whole army on 2 September. Yet this did not end the war, as the Third Republic was declared in Paris on 4 September 1870, and French resistance continued under the Government of National Defense and later Adolphe Thiers.
Over a five-month campaign, the German armies defeated the newly recruited French armies in a series of battles fought across northern France. Following a prolonged siege, the French capital Paris fell on 28 January 1871. Ten days earlier, the German states had proclaimed their union under the Prussian King, uniting Germany as a nation-state, the German Empire. The final peace Treaty of Frankfurt was signed 10 May 1871, during the time of the bloody Paris Commune of 1871.