The Franco-Prussian War was fought between the French under Napoleon III and the Prussians of Otto von Bismarck from 1870-1871. The war was provoked by Bismarck to further his agenda of creating a unified German State, rallying the southern German states to his cause who believed France to be the aggressor. A quick series of victories in by the Prussians and Germans in Eastern France lead to the capture of several key cities and Napoleon III himself, leading to the fall of the Second French Empire and the rapid creation of the Third French Republic.
The Republican forces were defeated as well culminating in the Siege of Paris. The war ended with the Treaty of Frankfurt in May 1871 and the result of the conflict was a unified German Nation State opening the door for German expansionism and Imperialism. France was ordered to pay significant reparations and to cede Alsace-Lorraine to Germany.
The combination of French bitterness and German nationalism resulting from the Franco-Prussian War planted the seeds of the larger First World War some 40 years later.