Louis understood that the larger a nation becomes the more secure it is likely to be. (Russia in the Cold War behaved according to that principle, disguising their true motive under the banner and slogans of communism). Louis also knew that a centralized government helps to bind a population into a cohesive unit something like a family. If every member of a national society understands that they are "French", for example, they will view their homeland more positively than will people whose nationality is indistinct, changing often with frequent invasions and "deals" made among princes and other types of power-brokers. Thus the French king eyed the small, neighboring German principalities on the west side of the Rhine as fair game in view of their feckless governance and the squabbling nature of the dukes and princes who fought one another constantly, seeking to enlarge their tiny territories. Louis' soldiers made short work of several such principalities, particularly Lorraine and Alsace.
Lorraine was and is a fertile place, ideal for growing crops and raising cattle. Alsace is fertile too, especially suited to raising the grapes that produce delightful, light table wines. With the support of the Church in those regions, Louis improved his situation simply by taking those two territories so poorly able to resist. It was the weaknesses - economic and military - of the small German states that Bismarck sought to improve with the Zollverein or "Customs Union" of the mid-nineteenth century. He knew how easily the vulnerable German states might fall prey to invaders - the French, the Russians. Once he had created the Zollverein, the German middle ages could truly be said to have ended. With the Zollverein also came the sense, among the German people, of belonging to a "nation" rather than a mere duchy or principality. That development alarmed the French who saw a unified Germany as a threat just as Germany, a century earlier, had seen a unified France as a threat. And so, somewhat more unified and very much stronger, in 1871 Germany took back what she had believed was hers when Louis took Alsace and Lorraine away from her. But there was another, perhaps deeper reason why Germany wanted the lost territory back.
Because the people of Alsace have long spoken a form of the German language, the Germans on the east side of the Rhine have felt a kinship with them. And that - simple as it is - was considered a justification for the longing of Germania to gather to her bosom the "Germans" of Alsace along with Lorraine which had once been a powerful German duchy. Until the Enlightenment, "Germany" was - from the English-speaking point-of-view - a word much like "Muscovy" and "Arabee", implying a roughly defined area where "germans" lived. Britain is another example. Until partly tamed by the Romans, it was a gaggle of warring tribes with no sense of belonging to a national "family".
National pride - French and German, Russian, British, Austrian - played a profound role in 1914, fanning patriotic flames among European nations now willing to fight for king and country. France wanted her lost territories back somewhat more fiercely than Germany had wanted them. Indeed, so strong was that wish that it became a French national obsession. To wrest Alsace and Lorraine away from the German Empire - created in Louis XIV's own palace! - would go far toward mitigating the shame of having lost them in 1871. There was no more urgent cause of the First World War than the French lust for revenge. Unless, of course, it was Austria's lust for revenge. More about "revenge" in the Franz Ferdinand and Count Berchtold sections.
Learn more about the causes of the World War I...