Excerpts from Books and Wikipedia
". . . the conflict was caused by Prussian ambitions to extend German unification and French fears of the shift in the European balance of power that would result if the Prussians succeeded. Some historians argue that the Prussian chancellor Otto von Bismarck deliberately provoked the French into declaring war on Prussia in order to draw the independent southern German states—Baden, Württemberg, Bavaria and Hesse-Darmstadt—into an alliance with the North German Confederation dominated by Prussia, while others contend that Bismarck did not plan anything and merely exploited the circumstances as they unfolded." [Wikipedia] "The magnitude and swiftness of the French collapse—by September 4 its battered army had surrendered at Sedan, Napoleon III was a prisoner, and the imperial regime had been overthrown in Paris—was a devastating blow to such rosy assumptions. As it turned out, neither Austria-Hungary nor Italy came to France’s aid, and French sea power proved totally ineffective. All therefore had depended upon the rival armies, and here the Prussians proved indisputably superior." [Kennedy: Great Powers, p. 186-7]