The Franco-Prussian War was the final stage in German unification. Now nationalistic fever also seized people in Germany.The catured French place of Versallies, King Wilhelm I of Prussia was crowned Kaiser or emeperor.
Bismarck was an expert at manufacturing "incidents" to gain his ends. He created the impression that the French ambassador had insulted the Prussian King. The French reacted to Bismarcks deception by declaring war on Prussia.