The Franco-Prussian War of 1870-1871. Emperor Napoleon III, under pressure from the press and "public opinion", declared war on Bismarck and King William I of Prussia, hoping to reclaim France's 1814 borders and prevent German Unification.

Prussia quickly overran France, capturing Napoleon III at the battle of Sedan 1870. Napoleon III was the last European ruler to command from the battlefield. Back in Paris, revolutionaries deposed the absent monarch and created the Paris Commune, burning down a significant portion of Paris, notably excluding the Rothschild's residence.

The German empire was proclaimed in the Hall of Mirrors at the Palace of Versailles, January 18th 1871. Alsace-Lorraine was issued a plebiscite where it voted to join the Second Reich. From 1871 to 1918, the French had a saying about Alsace-Lorraine: "Speak of it never, think of it always."