Franco-Prussian War
The ten-month war of 1870 that destroyed the French Second Empire, created Germany in the Hall of Mirrors, and planted the grievance that ran straight into 1914.
The Franco-Prussian War lasted from July 1870 to May 1871 and rearranged Europe more thoroughly than any conflict between Napoleon and 1914. France entered it as the continent's leading military power and left it defeated, occupied, financially indemnified, territorially reduced, and republican. Germany entered it as a collection of states and left it as an empire.
The provocation
Bismarck needed a war with France, and he needed France to start it. He had unified northern Germany after defeating Austria in 1866, but the southern Catholic states, Bavaria above all, were not going to join a Prussian-led empire out of enthusiasm. Only a common enemy would do it.
The pretext was a vacancy on the Spanish throne offered to a Hohenzollern prince, which France reasonably objected to as an encirclement. The Prussian king met the French ambassador at the spa town of Ems and politely declined to give further guarantees. Bismarck received the telegram reporting the meeting, edited it for publication so that the exchange read as a mutual insult, and released it. The French press and then the French government reacted exactly as intended. France declared war on 19 July 1870.
The war
It was decided in six weeks. The Prussian system, general staff planning, railway mobilization schedules, universal conscription with trained reserves, and breech-loading Krupp steel artillery, worked; the French system, brave professional soldiers, a superior rifle in the Chassepot, and no plan worth the name, did not.
Two French armies were surrounded almost at once. At Sedan on 1 and 2 September the encircled force surrendered along with Emperor Napoleon III himself. The Second Empire collapsed in Paris within two days and a republic was proclaimed.
The new republic then refused to quit, which surprised the Germans and extended the war by six months. Paris was besieged from September to January, its population reduced to eating the zoo animals, and improvised armies fought on in the provinces in what the Germans called a war of the francs-tireurs and answered with reprisals against civilians. The city surrendered in January 1871.
The settlement, and the wound
On 18 January 1871, before the war was even formally over, the German Empire was proclaimed in the Hall of Mirrors at Versailles, in the palace of Louis XIV, with the King of Prussia declared German Emperor. The location was chosen for exactly the reason it looks like it was.
The Treaty of Frankfurt in May took Alsace and most of Lorraine from France and imposed an indemnity of five billion francs, with German occupation until it was paid. France paid it early, which was itself a demonstration.
Bismarck's generals insisted on the annexation for strategic depth, over his own doubts about the political cost. He was right to doubt. Alsace-Lorraine made France a permanent, irreconcilable enemy, which is why Bismarck spent the next twenty years building an alliance system whose entire purpose was to keep France isolated, and why the Schlieffen Plan had to assume a two-front war.
Paris supplied a coda. The city, humiliated and armed, rose against the new national government in March 1871 and established the Commune, which was crushed by French troops in a week of street fighting that killed thousands more Parisians than the Germans had.
The echo
Three consequences ran directly into the twentieth century.
A united Germany in the middle of Europe, with the continent's strongest army and fastest-growing industry, was a permanent structural problem that neither its neighbors nor Germany itself managed to solve until 1945.
The revanche for Alsace-Lorraine became a fixed object of French politics, and the recovery of the provinces was a French war aim from the first day of 1914.
And every general staff in Europe drew the wrong lesson. The war of 1870 was short, decisive, and won by mobilization speed and offensive spirit, so the plans of 1914 were built to repeat it. They were built on the last war that had happened before the machine gun, barbed wire, and quick-firing artillery made the offensive suicidal.