1 September 1870, the bowl of Sedan in the Ardennes. Napoleon III is with his army. German armies close round both flanks and fire down from the heights. By evening the white flag is up; the next day the emperor and 100,000 men are prisoners. On 4 September Paris proclaims a republic.
Why then
Bismarck needs a war to bring the south German states into a Prussian Germany. In July he edits a royal telegram from Ems so that it reads like an insult, and Paris declares war on cue. Six weeks in, one French army is trapped in Metz; the other, marching to free it, is cornered here.
What they wanted
Napoleon hopes his dynasty survives the week. Moltke, the Prussian chief of staff, wants an encirclement. Bismarck needs a peace he can dictate and a war the German states fight together.
What changed
The empire dies here, but the new republic fights on. Paris is starved through the winter; on 18 January 1871 the King of Prussia is proclaimed German Emperor in the Hall of Mirrors. France gives up Alsace and part of Lorraine. In spring the Paris Commune rises and is crushed in a week of killing.
Still argued
Did Bismarck want the provinces, or did his generals force them on him? And did taking them make 1914 inevitable? Historians have argued both ever since.