Britannica Sources used in this answer

Why did Napoleon III declare war on Prussia in July 1870?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Portrait of Napoleon III in a dark military uniform with gold epaulettes, a red sash, and medals, set against a deep red background. Painted by Hippolyte Flandrin. — Napoleon III Napoleon III, detail of a portrait by Hippolyte Flandrin, 1862; in the Versailles Museum.

Napoleon III declared war on Prussia on July 19, 1870, primarily driven by a desire to check Prussia’s growing power and to restore the fading prestige of his own empire through a military victory.12

The immediate catalyst for the conflict was the candidacy of Prince Leopold of Hohenzollern-Sigmaringen for the vacant Spanish throne. France feared that a Hohenzollern monarch in Spain would result in a dangerous encirclement of French territory by Prussian influence. Although Leopold’s candidacy was withdrawn following French diplomatic pressure, the French government demanded a guarantee from King William I of Prussia that the candidacy would never be renewed.123

When the King refused this demand, Otto von Bismarck edited the report of the encounter—known as the Ems telegram—to make it appear that both the King and the French ambassador had insulted one another. The publication of this provocative version inflamed public opinion in Paris, leading Napoleon III to declare war, confident in the superiority of the French army and its technical innovations.132