I just finished Geoffrey Wawro's book on the Franco-Prussian War, which I thought provided a wonderfully clear explanation of the causes and course of one of the nineteenth-century's most important conflicts. I'll provided a more detailed review later on, but in essence he attributes the outcome to two complimentary factors: the superior preparations of the Prussian military, and the failure of French leadership.
It is the latter of the two that I found especially striking. On the surface France had a lot going for it: the dominant power in Europe for over two centuries, it was a prosperous country with a large army that was well experienced in warfare. Moreover, having observed Prussia's victory in the Austro-Prussian War just four years before, they knew how the Prussians waged war and were adapting accordingly. Yet these factors were offset by the rot at the heart of the French leadership, from an ailing and disengaged emperor to generals more interested in fighting among themselves than battling the Prussians. This I knew; what Wawro also includes is the disaffection of the French populace from not just the empire but from their army, which was a collection of aging careerists disassociated from the larger population. The combination proved fatal when they went to war against Prussia in 1870, and while I hate to read the past into the present I was struck by some of the lessons it had to offer for today.