The Franco-Prussian War is a war between France and Prussia. Usually Prussia is the aggressor, and it tries to annex Alsace-Lorraine, but France can also be the aggressor.
Historically, the war was used to unite Germans into fighting a short war against 
France. The war successfully united the Germans into a single, unified German state, 
Germany.
The war in real life triggered when 
Prussian prince Leopold of Hohenzollern-Sigmaringen was offered a candidacy to the Throne of 
Spain. Through the fear of Prussian-Spanish alliance, France put pressure on the candidacy of the prince and thus the candidacy was withdrawn. However, Otto Von Bismarck altered the telegram sent by the king of Prussia and encouraged the idea of war onto the French. After nine months, the French Empire lay dead and there emerged a German Empire!