How did the Franco-Prussian war help unify Germany?

It gave the German States / Kingdoms an excuse to rally around Prussia's tiff with France, giving them a common enemy and a common purpose. Here is a more eloquent assembly of words.

http://www.bartleby.com/65/fr/FrancoPr.html

"""The emergence of Prussia as the leading German power and the increasing unification of the German states were viewed with apprehension by Napoleon III after the Prussian victory in the Austro-Prussian War of 1866. Bismarck, at the same time, deliberately encouraged the growing rift between Prussia and France in order to bring the states of S Germany into a national union. He made sure of Russian and Italian neutrality and counted—correctly—on British neutrality. War preparations were pushed on both sides, with remarkable inefficiency in France and with astounding thoroughness in Prussia. 2

The immediate pretext for war presented itself when the throne of Spain was offered to a prince of the house of Hohenzollern-Sigmaringen, a branch of the ruling house of Prussia. The offer, at first accepted on Bismarck’s advice, was rejected (July 12) after a strong French protest. But the aggressive French foreign minister, the duc de Gramont, insisted on further Prussian assurances, which King William I of Prussia (later Emperor William I) refused. Bismarck, by publishing the famous Ems dispatch, inflamed French feeling, and on July 19, France declared war. 3

The Course of the War

Partly because they believed France the aggressor, the states of S Germany enthusiastically joined the North German Confederation—just as Bismarck had hoped. The military conduct of the war was, for the Germans, in the hands of Helmuth Karl Bernhard von Moltke, a military genius. On the French side, Napoleon III took active command, but it soon devolved on Marshal Bazaine. ""

Peace...

Ok, let's think about it.

If a country is in a war against a common enemy, mainly a popular war, does that unify or divide a country? You and your neighbor might be very different people, but then here comes this external power that you both are fighting against. Wars build patriotism, and patriotism makes a people feel more united. Us versus them.

So Germany (or more specifically, Bismarck) started a war with France, in the hopes that all of the outlying German states would join up with the rest of the German people into one nation. Look up the war for more specifics.

Well, Prussia and her German allies won. After their victory, they proclaimed a united German Empire in the Hall of Mirrors in Versailles, Paris.

Because the Germans beat the pants off the French.