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Why did France declare war on Prussia in 1870?

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Otto von Bismarck in military uniform and cap, with a mustache, facing slightly to the side in a black-and-white portrait taken around 1880. — Otto von Bismarck Otto von Bismarck.

France declared war on Prussia on July 19, 1870, driven by a desire to check Prussia’s growing power and restore the prestige of Napoleon III’s empire.1

The immediate catalyst was the candidacy of Prince Leopold of Hohenzollern-Sigmaringen for the vacant Spanish throne. France feared that a Hohenzollern monarch in Spain would create a strategic alliance between Prussia and Spain, effectively encircling France. Although Leopold withdrew his candidacy under diplomatic pressure, the French government demanded that King William I promise never to allow the candidacy to be renewed.2

Otto von Bismarck manipulated the situation by editing the Ems telegram—a report of the King’s meeting with the French ambassador—to make it appear that the King had insulted the ambassador. This provocation successfully goaded the French government into declaring war. Napoleon III’s military advisers, confident in the superiority of the French army, believed a victory would bolster the Emperor's declining popularity at home.123