Battle of Sedan
The Battle of Sedan was the most important battle of the Franco-Prussian War. Napoleon III personally led the French army to the frontier area around Sedan. There he was encircled by the Prussian forces. A French cavalry effort to the north failed to break out of the trap. Hundreds of thousands of French soldiers, including the emperor himself, were taken prisoner.
Following the battle the remaining French attempted to rally, but the Prussians advanced to the outskirts of Paris. Then an uprising in Paris led to the short-lived Paris commune.
These events were written about by Emile Zola in his novel, The Debacle.