Franco-Prussian War
The First World War was an unimaginably brutal conflict. Some of the worst fighting of the war took place between France and Germany. Leading up to the war, the two countries had experienced decades of tension. In order to understand the entirety of World War I, we need to go back a few decades to the Franco-Prussian War and the resulting unification of Germany.
So it's really important to remember that Germany did not become a nation-state until 1871. Before that, what is now Germany was composed of numerous independent Germanic kingdoms-states, such as Prussia, Bavaria, Saxony, and others. Throughout the nineteenth century, tensions periodically flared between France and some of the Germanic states on its eastern border. By the 1860s, both the French and the Germanic states recognized that, sooner or later, war was coming. The Franco-Prussian War was fought between France and what is now Germany between 1870-1871.
The exact causes of the war are complex (like much nineteenth-century European history tends to be), but let's see what we can do to quickly highlight the basics. So basically France was no dummy, and could see that over the course of the nineteenth century the Germanic states were increasingly gaining power. France became concerned when a Prussian Prince named Leopold of Hohenzollern became a candidate for the throne of Spain. See, France was concerned that if he became King of Spain, a Prussian-Spanish alliance would be forged, resulting in France being surrounded by unfriendly states.
So what did France do? France sent diplomats to try to talk the Prussians into withdrawing Leopold's candidacy. Here is where the juicy part comes in. Otto von Bismarck, the Prussian Chancellor and wily statesman, altered a letter from Prussian King Wilhelm I, making it sound like the King had insulted the French diplomats. Bismarck then released the contents to the public. The French population, believing that King Wilhelm I had egregiously offended their diplomatic envoy, was enraged and demanded war to regain national honor. On top of this, Napoleon III of France was pretty unpopular at this time, and he felt that war would help him gain favor with the French people.
The French believed their highly professional army would achieve a swift and decisive victory against the upstart Prussians. This was not to be. See, what France underestimated was the spirit of unity growing among the Germanic states. After France declared war on Prussia, some of the other Germanic states joined in against France. This was Bismarck's plan all along. Most historians believe Bismarck's purpose for provoking the Franco-Prussian War was to bring national unity to the various Germanic states.