French aspirations for prestige and Bismarck's (chancellor of Prussia) desire to unify Germany under Prussian leadership set the stage for the war.
The immediate cause for hostilities was an effort by Bismarck to secure the Spanish throne for his candidate which threatened France with a combination of Spain and Prussia directed against it.
Bismarck further infuriated France with a provocative message (the Ems telegram) and Napoleon III, the French emperor, declared war. Within four weeks the Germans under the leadership of General Helmuth von Moltke encircled the French army in Metz.
After Napoleon III was taken prisoner, a republican army continued to fight until January 1871 when Paris capitulated.
As a result of the war King William I of Prussia was proclaimed German emperor, French prestige declined, and a period of 40 years of unstable peace between the major powers of Europe ensued.