The humiliating defeat of Louis Napoleon’s Second Empire of France is made complete on May 10, 1871, when the Treaty of Frankfurt is signed, ending the Franco-Prussian War and marking the decisive entry of a newly unified German state on the stage of European power politics, so long dominated by the great empires of England and France.

At the root of the Franco-Prussian conflict was the desire of the ambitious statesman Prince Otto von Bismarck to unify the collection of German states under the control of the most powerful of them, his own Prussia. The event that immediately precipitated the war was the Bismarck-engineered bid by Prince Leopold, of the Prussian Hohenzollern royal family, for the throne of Spain, left empty after a revolution in 1868. Horrified by the idea of a Prussian-Spanish alliance, the French government of Louis Napoleon (or Napoleon III) blocked this idea and, determined to humiliate Prussia into subordination, insisted that the Prussian king, Wilhelm I, personally apologize to the French sovereign and promise that there be no further such attempts by the Hohenzollerns. Wilhelm refused, and subsequently authorized Bismarck to publish the French demands and his own rejection of them; the prince did so knowing such a move would precipitate a war, which he himself greatly desired in order to free Prussia completely from French influence.

Eager to regain prestige after numerous defeats abroad and reassert its military dominance on the European continent, France declared war on July 19, 1870. Unfortunately for the French, the states of southern Germany honored their treaties with mighty Prussia and immediately backed Wilhelm’s armies. Thus the Germans were able to marshal some 400,000 men, double the number of French troops, at the outset of the war. Under the supreme command of Wilhelm and guided by Count Helmuth von Moltke—known as Moltke The Elder, to distinguish him from his nephew, who would command German forces during World War I—three German armies cut a broad swath through France, gaining the upper hand almost from the beginning of the fighting.

The crucial battle of the war, fought around the town of Sedan in northern France, resulted in a crushing German victory, in which Napoleon III himself was captured. Upon learning of the emperor’s capture, Paris exploded into rebellion; the legislative assembly was dissolved, and France was declared a republic. Meanwhile, the Germans were closing in: by the end of September, they had captured Strasbourg and completely surrounded France’s capital city, which they subjected to merciless siege and bombardment for the next several months. On January 19, 1871, the French government was forced to open negotiations for surrender. A day earlier, in an added humiliation for France, the Bismarckian dream of unification was fulfilled, as Wilhelm I of Prussia was crowned emperor, or kaiser, of the new German state, in a ceremony that took place in the sumptuous Hall of Mirrors, at Paris’s Versailles palace.

By the terms of the final treaty, signed on May 10, 1871, at Frankfurt am Main, Germany annexed the French provinces of Alsace (excluding Belfort) and Lorraine; the French were also ordered to pay an indemnity of five billion francs. German troops occupied France until September 1873, when the amount had been paid in full. The Franco-Prussian War and the nearly three years of German occupation that followed marked the beginning of a growing enmity between anxious France, its influence and power in decline, and striving Germany, a technologically and industrially superior nation that by the first decade of the 20th century had built the most powerful land army on the European continent. In the summer of 1914, this rivalry would explode into full-scale global warfare, pitting France and the Allies against Germany and the Central Powers in the most devastating conflict the world had yet seen.