The Franco-Prussian War - known in Germany as the German-French War and in France as the Franco-German War (Guerre franco-allemande) - was the last of three wars that led to the unification of Germany (the first since the Holy Roman Empire had any political power).
There were multiple causes of the war, including but not limited to, a potential sale of Luxembourg to France, the vacancy of the Spanish throne, and the Prime Minister of Prussia modifying and publishing an insulting telegram- about a meeting of the French Ambassador. For whatever cause, a dangerously under-prepared France declared war on Prussia (and thus the North German Confederation) in July of 1870. These circumstances led to the South German states (Bavaria, Baden, Wuerttemberg) to join the side of the North German Confederation.
The result was a 10-month Curb-Stomp Battle as the Prussians decimated the French in all but three battles (where the French won one at Broney-Colombey and fought to a draw in two others), captured the French Emperor, Napoleon III, and unified Germany.
Another result of the war was Germany's annexation of Elsass-Lothringen (Alsace-Lorraine), which they held until World War One. Moreover, the French had to leave Rome, indirectly finishing the Unification of Italy.
The defeat of Napoleon III led to the proclamation of the Third French Republic, which continued the war longer than the Germans expected by continually raising new armies even as Paris was besieged. In a bloody epilogue after the signing of the definitive peace treaty French fought French as the forces of the conservative central government put down the Paris Commune.