Franco-Prussian War:

The causes of the Franco-Prussian War are deeply rooted in the events surrounding the unification of Germany. In the aftermath of the Austro–Prussian War of 1866, Prussia had annexed numerous territories and formed the North German Confederation. This new power destabilized the European balance of power established by the Congress of Vienna in 1815 after the Napoleonic Wars. Napoleon III, then the emperor of France, demanded compensations in Belgium and on the left bank of the Rhine to secure France's strategic position, which the Prussian chancellor, Otto von Bismarck, flatly refused. Prussia then turned its attention towards the south of Germany, where it sought to incorporate the southern German kingdoms, Bavaria, Württemberg, Baden and Hesse-Darmstadt, into a unified Prussia-dominated Germany. France was strongly opposed to any further alliance of German states, which would have significantly strengthened the Prussian military.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What established the European balance of power in 1815?
the Congress of Vienna