Franco-Prussian War, 1870-1871.

French Artillery crew during the Franco-Prussian War, 1870-1871, France
-
The Franco-Prussian war is left out of nearly every history textbook or timeline of important European events, but in my opinion, was probably the most crucial event in the formation of 20th Century European history. The war itself was extremely short, only about 6 months, lasting from July of 1870 until January of 1871, when the German confederation captured Paris. The war stunned neutral onlookers, who had expected not only a long war but a French victory, and eventually led to the formation of the German Empire. (Deutsches Kaiserreich) 

After the Austro-Prussian war of 1866, the Kingdom of Prussia had annexed numerous territories and formed the North German Confederation, causing a destabilization of power in Europe formed after the Napoleonic Wars. After this, Prussia and Otto Von Bismark turned their sights south, in an attempt to unify the southern German kingdoms of Bavaria, Württemberg, Baden and Hesse-Darmstadt, into a unified Prussian-dominated Germany. France strongly opposed this, as it would give much more military power to the Prussians. 

The immediate cause of the war resided in the candidacy of Leopold of Hohenzollern-Sigmaringen, a Prussian prince, to the throne of Spain. The French feared an encirclement of Spainish and Prussian alliance. French pressure withdrew Sigmaringen's candidacy, but Otto Von Bismarck baited the French into declaring war, by releasing an altered summary of the Ems Dispatch, a telegram sent by William I, rejecting French demands that Prussia never again support a Hohenzollern candidacy. "This text produced the effect of a red flag on the Gallic bull", Bismarck later wrote. Following the declaration of war, the Southern German states immediately sided with the Prussians.